Paper 5. Friday 25th March 2016 DISCUSSION ZONE

DO NOT read other peoples comments until you finish posting yout comments. Or you will pick up bias. Feel free to read them after you post your comments.

After completing the questions, post your comments below NOT FORGETTING TO LEAVE YOUR NAME !!!!. Tell about difficulties faced and what you found tricky, as well as any ‘clever tricks’ or good thoughts you employed in answering these questions.

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34 thoughts on “Paper 5. Friday 25th March 2016 DISCUSSION ZONE

  1. Dear Kychem,

    Regarding the paper that was given out today, I ran into a few problems.

    First of which was 51/MJ/14 2. c) as well as 51/ON/14 2. b)
    In the first one, there weren’t any anomalous points, so I wasn’t to sure on how to explain it other than saying that “all points plotted lie on or near to the line of best fit” which wouldn’t get me the 2 marks. In the second one, it wasn’t a straight line graph, so I wasn’t too clear on which points to ‘ignore’ and as a result I drew through all the points. Apparently, the one at 12 cm was considered an anomalous point. But it would barely look off.

    For 51/MJ/14 2. e)
    I already clarified this with Mr Allan earlier, however I figured I’d bring it up here anyways. In the question, it wasn’t stated which one was oxidised, hence to get the SEP for M, we could either do M – 0.8 = 0.93 or 0.8 – M = 0.93 both would be valid since both will allow us to get the E cell. Evidently, 0.8 – 0.93 gets us -0.13 V which corresponds to Pb in the data booklet which makes more sense than the alternative of +1.73 V which doesn’t really get us anything. Mr Allan said we should take which ever makes more sense.

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  2. Things I realized….
    1) Accidentally miscalculating the grad as x/y instead of y/x (careless mistake)
    2) What explanation to give when there are no anomalous points, “variations due to rounding”
    3) Ions and NOT electrons move across the salt bridge (anther careless mistake)
    4) The substance used for making a salt bridge must not cause precipitation
    5) A way to control heat was by controlling the strength and distance
    6) Question (g) on 51/mj/11 left me blank… and also question (d) on 51/on/14.
    7) Commonly asked question “Even though exp was done carefully with accurate apparatus, why is there still error?”
    Generally accepted ans:
    -Measurements and readings have error
    -graph drawn by hand is not accurate
    -exp was not repeated
    However, do refer back to the question if the ans can be applied.

    – Ezzaty Hasbullah –

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  3. Difficulties I faced
    – I was unsure how to explain the lack of anomalous points, being familiar only with the reverse argument, i.e why there are anomalous points; although in the last question I was unsure if any other logical argument would get me the marks aside from the typical ‘added chemical was not mixed properly’
    -I took more time than I should have before I reasoned out why the standard electrode potential of M was a negative value and not a positive one
    -I was also unsure if my conjectures as to why the results of an experiment is still subject to error were satisfactory

    Mohammad Hariz

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  4. The first question was relatively straightforward except for part c where I was unable to give a reason for the absence of anomalous points. Apart from that, the graph and the calculations were simple.
    The second question was fine as well except that I was not completely sure whether the diagram I drew for the apparatus was completely accurate.
    I did not have any other problems with the rest of the paper.

    Eugene

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  5. Assalamualaikum Sir, First of all, i seek for apologize for submitting this responses one day late as i am struggling to finish all question. Well, I don’t have much suggestion rather i have several question which had made my work slow.

    These are the question:

    M/J/51/14

    1. 2b) How do we exactly make a best fit line? Do we have to consider the overall points as in the dots and make a balance line ( although the line doesnt touches the dots at all??). For instance, there are 8 points in the graph and I make a line with 4 dots above and 4 dots below (the line doesnt touches any point at all). Is this even possible?? Another one is Lets say we have 7 dots in total, how do we get a best fit line?? do we just make line touching a dot and make it 3 above and 3 below the line???

    For this question, i made a line with 2 points below the line and 1 point above the line and other points touches the lines so, (does this make my line not best fit)???

    2c) [regarding the graph i made] can i just circle one point that doesnt touches the line as anomolous point thus make it a balance with one point above and another below the line????

    is there any conditions to justify the point as anomolus??/

    2gii) haha, sorry sir for asking you this. why cant we use KCl and what makes it different from KNO3 and K2SO4???

    M/J/11/51

    1b) do we have to state values (if any) for the independent and dependent variable??

    1d) Overall, for this question can i just make a whole steps for this experiment or shud i just state points particularly in i,ii, and iii???

    diii) can i suggest heating the carbonate in water bath at 60 degree celcius? or using thermostatically-controlled water bath? (is there any of this in the lab? i see one in biology lab though)

    1e) is there any hazard sir? is Co2 hazardous??

    1g) actually i was thinking to do the experiment in temperature controlled lab, can i??

    O/N/14/51

    e) actually i was very confused whether to use the ka= [NaA]/[HA]x[NaOH] if so, how to find their concentration??? or the pH=pKa+log[salt]/[acid]

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  6. Here are few findings and doubts from the paper:

    1. When heating carbonate. is direct heating recommended or water bath needed to ensure even heating?
    2. For the explanation for the rate of decomposition, what needs to be mentioned? Because my explanation is too long I guess for just 1 mark.
    3. I also could not identify the hazard in the decomposition experiment.
    4. I also do not really understand the questing asking about controlling the factors in heating
    5. Will I be penalized for my bad graph sketching? Because my curve was not really smooth at first. I had to redraw to get a smoother one.

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  7. Difficulties I faced
    1. Identifying anomalous points. Probably due to severe lack of practice. Especially with graphs like the titration curve in 9701/51/O/N/14. What’s up with that? ._.
    2. Lost marks for not being specific. E.g. Which (of the following) salts are unsuitable for a salt bridge? The answer was such that salts with Cl- and SO4(2-) were unsuitable as they precipitate with Ag+ and Pb2+ respectively, but I just wrote that they ‘react’ without specifying what exactly happens. (9701/51/M/J/14)
    3. Question 1(a)(i) from 9701/51/M/J/11 asked to predict how the rate of decomposition of the group II carbonates will change as the group is descended, but the answer given (for prediction) stated to predict that they decompose less easily. Still not sure what’s going on there.
    4. VERY ANNOYED WITH PLANNING QUESTIONS. Wrote a whole page of the process to find out that I only got 1 out of 4 marks e_o T_T. It seems like all you need to do is state what they asked for in the 3 or 4 parts and ignore the rest of the process. Oops.
    5. Improvement in apparatus requires lots of practice and experience it seems. Will look into more past year answers for such questions.

    All in all, paper 5 is alright, but it seems like familiarising yourself with the format is absolutely essential not only for getting the right answers but for finishing within the time limit.

    Azizan Wazir, Block 4.

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  8. Hello,
    From questions , May/june(51) 2014
    Q(c) -I cannot really figure out the anomalous points. What is the exact characteristic of anomalous points? Do we have like a specific measurement for this points?
    Q(g-ii) I cannot remember that all nitrates is soluble in water. So it’s kind of hard for me to analyse this question properly, which one going to produce a precipitation with pb.

    May/June (51) 2011
    Q1 a(i) for the explaination part, I explained about the correlations between the charge density of group 2 decreases as the atomic radii increases as decends the group. Hence the power of polarising carbonate ions decreses. But in the mark schemes, it relates with Mr . Also, for the graph sketching part, i sketched rate of decomposition against charge density instead of Mr. Is my answer acceptable?

    Oct/Nov (51) 2014
    (e) I found this question a bit tricky as I cannot derive an equation that relates with concentration and the volume , (M1V1=M2V2). So it is a bit hard for me to find the ratio between two concentrations.
    (F) one of the tricky part is, I forgot to make the assumption between the concentration of H+ and A- ,it should be the same.

    Syafiq Izuddin

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  9. What I’ve learnt:
    (a)51/M/J/14 If there aren’t any values that are anomalous, the differences are most likely due to rounding instead of experimental errors.
    (b) 51/M/J/14
    Question (e) required the standard electrode potential for M to be calculated. It seems like there could be two possible values, one in which the value for M is more positive than Ag and another for which the value for M is more negative than Ag. The way around this problem is to consider the fact that the cell potential increases as the concentration of ions of M decreases, indicating that the standard electrode potential for M should be negative. If it were more positive than Ag, the cell potential would decrease with decreasing concentration of ions of M.
    (c) 51/O/N/14
    In question (e), it seems at first sight that there isn’t enough information available to calculate the dissociation constant of the weak acid. However, information from part (d) comes in handy. The ratio of concentration of base to the weak acid can be calculated be equating the number of moles of base and acid needed for complete neutralization. Once this ratio has been calculated, the formula that relates pH and pKa can be used to calculate the Ka of the weak acid.
    Haemish~Block 5

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  10. Very good comments. Thank you.
    These questions are frankly pretty mean/cut-throat. I will try and give commentary as to what you have written above, or reply specifically if you have a unique point.

    To those who have not done the required work, except Faizah, Law May and Henry, I will give you until the end of Wednesday 30th March to submit your comments (and I expect committed intelligent comments, not piecemeal efforts). Failing that, you will have to type out all answers including scanned graphs and sketches etc,. and e-mail me before the end of Friday.1st April.

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  11. here are few difficulties i faced when answering this question papers:
    1) 51/m/j/14
    – for question (c) , there is no anomalous point but i don’t understand why the reason for that is variations in points due to rounding
    -can u explain why only potassium nitrate is suitable to use for salt bridge and why not KCl and K2SO4?

    2)51/m/j/11
    – for question (a)(i), can u explain how rate of decomposition decreases down the group? i can’t figure it out as the mark scheme only stated higher Mr compounds decompose less easily and polarisation decreases. How higher Mr can cause polarisation decreases?
    – for the design question (d)(ii), to calculate mass of carbonate,, is it initial mass – final mass ? how to compare the mass lost from the heating with the volume of CO2?
    – for question (g), i used syringe to collect the CO2 gas , is there any alternative apparatus for this experiment which is better than syringe? and how to improve this experiment if i didnt change the apparatus?

    3)51/o/n/14
    -for question (c), i wrote 6.5-10.6 for the range of pH values as that’s what i got from my graph. but the mark scheme stated 6.5-11.
    is my answer acceptable or not? or the range is depends on how i draw my graph?

    That’s all. Thank you,sir. =)

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  12. M/J 14 51
    c) I took it for granted that there will always be anomalous points in a graph because i remember doing one past year where the points did not differ much but was considered anomalous. (can’t remember which year and I may have made an error in plotting)

    d) I did not understand how the equation was related to the graph until it was pointed out the eqn is a y=mx+c graph. Instead, I substituted the values from a point into the eqn to find n instead of using the gradient.

    M/J 11 51

    e) Why is open flame from a bunsen burner not considered a hazard? (mark scheme only refers to hot apparatus and not the equipment)

    f) if we add extra columns for other info that is not in the mark scheme into the table, would we be penalised?

    O/N 14 51

    d) Is [H+] the same as [A-] at half equivalence point? Because from the eqn, I thought they dissociate to the same extent so concentration should be the same?

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    • Oh and also I found it tricky to think of improvements and errors. So basically
      Improvements: in terms of reducing random errors and minimising % errors
      Errors: coming from random errors and assumptions not being true

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  13. Hello sir…

    For MJ14(51)
    – the graph is very confusing. At first I didn’t understand why the MS said that there’s no anomalous points. Because from the graph that I drew, there are 2 extra points that are not on the line. After consulting some of my friends, I understand that it is because we have rounded up the value earlier, and the points are actually very close to the graph, so they are not anomalous points.
    – about the salt bridge, i just knew that we need to make sure the salt that we used must not react with the anion & cation.

    For MJ11(51)
    (d) when the question ask how to control the factors in the heating, i thought we need to use water bath. But in the MS it is stated that water bath is not allowed. Why is it sir?
    So, based on the ms, ways to control the heating is by using the same settings on the bunsen burner (how to actually make sure that the setting is the same?) and the bunsen burner must be at the same distance.

    For ON14 (51)
    – (e) I intially did the qs using the Ka formula, then realized that I didn’t have enough information. So I used pH=pKa. But sir, i thought we can only use pH=pKa at the equivalence point only? Based on the qs, at 15cm3, it is not the equivalence point.

    These are the difficulties I encountered when doing the papers. Apart from these, I also did some careless mistakes, which I’ll try to avoid during the exam. 😁

    Aishah Nadia Block 1

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  14. Hi sir ,

    9701/51/m/j/14
    It’s my first time seeing a question w no anomalous point so it’s quite confusing at first. Others than that,it’s ok for this question.

    9701/51/m/j
    Spent quite some time to think of the hazard and precautions should be taken. Also the alternative apparatus to improve the experiment. (Need to do more practice, I guess 😰) and also the tabulation. It was quite confusing at first .

    9701/51 /o/n/14
    Can I know the right to do the calculations because I’m not sure if I do the calculations part correctly.

    I think that’s all. Thank you

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  15. Here’s what I learnt,

    51/MJ/14
    (a) Beware of the significant figures of the value calculated from raw data, in this case 2/3 sig fig is enough.
    (c) A rather special one with no anomalous point. I didn’t know variation due to rounding could be an answer, something new here.
    (d) iii, Need to be special aware of the sign here, or else will get it wrong.
    (e) Not sure if we can use the formula introduced in textbook, Ecell=Eright-Eleft. But by analysis, it is quite unlikely for any metal to have a more positive E value than Ag. Therefore you should get an answer less than the E value of Ag, in this case, Pb will be the one.
    (f) Remembering mass and charge must be conserved in a regular chemical equation, any equation can be written with ease.
    (g) ii Recall the solubility rule is important, a reminder to myself not to forget the basics.

    51/MJ/11
    (a) i, No part of the question ask us to refer to the Mr of the molecule and comment on it. I’m not sure by just stating the prediction will earn any marks. Again, we need to relate the physical properties to chemical effect to earn marks for explanation, a typical technique examined throughout the Chemistry paper.
    (b) For one mark question, it is best to give short answer, giving long answers open rooms to error, which might in turns causing marks to be deducted.
    (c) Remember to specify the size of the collecting instrument.
    (d) Learn that many lines provided not necessarily means that you have to use all of them. iii, is something new to me, it seems Paper 5 answer sometimes are just simple answer by common knowledge or basic Chemistry.
    (f) Guide to good table, always have column for raw data, and then only add more for calculated data. Be aware that sec is not accepted in place for “second” or you could just have write “s” instead.
    (g) This is new to me, didn’t know that electrical hot plate can be used too. Never think of the solubility of CO2 when passes through water. Just a question here, what if my initial set up is already good enough as they suggested? i.e. In the earlier part I drew a boiling tube connecting to a gas syringe? Should I deliberately make the planning on the first part to be less perfect so that I can answer the later part easier? The key to this kind of question is to change to something that you have greater control over I deduce.

    51/ON/14
    (b) Reasons provided by the answer often involve “incomplete reaction” kind of pattern. Identify the causes to incomplete reaction shall help to answer most of the question of this kind.
    (d) ii, having the knowledge that pKa of a weak acid can be determined from the graph by getting the value of pH when the titration is half done, i.e. v/2, helps to answer this, as the assumption is that [A-] and [HA] is approximately equal.
    (g) This kind of question I dislike the most due to the large number of lines provided but only 1 mark. Not sure what kind of answer to give. But it seems all boils down to a similarity, question of this kind usually wants us to comment about the practicality of the experiment, i.e. human error/systematic error that is not being able to be eliminated fully (uncertainties, uncontrollable fluctuation in temperature (at least in school lab) etc. ) Any assumptions or approximation used could be source of error too. The reason suggested in the mark scheme like “graph drawn by hand is not accurate and experiment is not repeated” is something new to me. Just wondering should I give more reasons in the real exam because this question doesn’t seem need to recall factual knowledge, the likelihood of marks being negated is low, so… .Hmmm

    That’s all from me.
    – Li Hong (Block 1)

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  16. Hellooo Sir!

    Having done the questions,I realised that I have some queries that I wish to clarify.
    =D
    When questions asked to suggest or calculate the amount of reagents we should use in the experiment, do we have to standardize the no.of moles of reactants or their masses?And
    I understand that the use of Data Booklet is unnecessary(as stated on the front page of the paper).If that’s the case, do we have to specify the masses used for the reactant (as in Question 1(d)(ii) from s11 qp51) or we just have to include a general formula of the calculation?The third quest is do I have to show my raw data in the table if I’ve already done so when writing my procedure.

    These are the problems I always encounter when it comes to planning an experiment and I hope that my questions will be answered soonnn=)

    Thank you Sir and have a nice dayyy!
    Jingyi (Block 4)

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  17. Dear sir,

    For June 11/P51,
    I realized that the question paper specifies that we need to specify a certain value for our experiment, however if not specified, do we have to specify numerical values as well? Is it safe to just assume that all experiments require us to specify reasonable values when we write our procedures?
    For the factors of heating, is it fine to put down ‘conduct experiment in room with stable air flow’ so that the Bunsen burner will burn at a steady rate?
    I realized that typically for those types of questions, I can’t easily deduce the answers in the mark scheme, is there any possible way to effectively deduce the keywords?

    For Nov 14/P51,
    I didn’t get an anomalous point for the graph I plotted. It seemed really fine to me but the ensuing question demands me to state the anomaly points, and I’m really, really confused.
    Also, in general, question 2 always ask us about reasons for the occurrence of anomaly, for this is there anything you might advise on?
    The suitable range of pH values lies between 6.5-11, however is it fine if I put it s 6-8? Is there a logical reasoning behind those range of values?
    In (d), how could I relate the volume to the number of moles of each reactant? I tried using MV/1000 but I am still stuck (mark schemes only provided the answer so I can’t really properly determine the moles)

    For June 14/p51,
    I thought this was decent as the instructions given are clean and straightforward. I kinda got carried away into thinking if the instruction to insert the number of decimal places are not given, is there a general rule of thumb for either recording values in data according to dp or sf if there is any time when instructions regarding dp/sf are not given?

    I think that’s all from me for now, I’d appreciate it for any advice!
    Thank you! And have a good day!

    -Zhuhan-

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  18. Salam Sir!

    I’ve run into a few difficulties when I did the 3 questions.

    M/J 2014 51
    1) I just knew that anomalous points mean points that are at a considerable distance from the straight line. And the variation in points are due to rounding to 2 decimal places.
    2) Since the other half cell uses a metal electrode, it must’ve undergone oxidation. Thus the overall cell potential must be negative (careless mistake).

    M/J 2011 51
    1) For d, I thought that the mass had to be constant and did not realise that actually the number of moles had to be constant.
    2) For how to control the heating apparatus, I mentioned that the air-hole of the Bunsen Burner must be fully opened to ensure that the strength of the flame would be constant. Would that suffice?
    3) I had a problem figuring out the improvement to my apparatus but only just realised that I should change my conical flask to a boiling tube (a smaller measuring apparatus). Can you explain any other improvements to be done?

    O/N 2014
    1) I don’t really understand why ph = pka at 15cm3. But I did it anyway because the was no other method.
    2) The concentration of the ions was also quite confusing.
    3) Can you explain more about the errors posibbly made during the experiment? i had a hard time during this equation too.

    Hani Irfan (block 1)

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  19. Sorry for the delay, sir..Here’s the work you requested.

    Below are some of the difficulties I faced when attempting the paper:
    2014/m/j/51
    2(c) I couldn’t think of reasons for anomalous points. I was misled by the question into thinking that there should be an anomalous point.

    (e) As the E=+0.931V, I assumed that the M+ electrode was the positive terminal. (Since it was stated that Ag electrode is used as reference electrode in place of SHE, so I thought that the value of Voltmeter reading will give the positive/ negative terminal as well)
    Because of that assumption, I tried to calculate the Std Electrode Potential of M by using 0.80V+0.931V. Obviously, I couldn’t find the identity of M.

    A way to go about this: most metals are better reducing agents than Ag, so their standard electrode potential should be more negative than that of Ag. Following that, I was then able to find the identity of M.

    2011/m/j/51
    1(e) For hazards, I talked about the possibility of the boiling tube to break during heating. So I suggested the use of safety goggles and lab coats to prevent injury from broken glass. Was not awarded the mark.

    (g) Had difficulty thinking of ways to improve reliability. I suggested a few ideas but wasn’t sure if they will be credited:
    – Saturate water with CO2 by bubbling CO2 for 30 minutes to combat the problem of gases dissolving in water during experiment. (I later realised that this will probably be rejected as the question asked for improvements in “apparatus”)
    – Use a gas supply instead of canned liquified gas to ensure uniform gas supply for uniform heating temperature.

    2014/o/n/51
    2(c) The range of pH values I gave was too broad.

    (g) I had difficulties suggesting possible errors during experiment.
    My answers were not given in the mark scheme and I wasn’t sure if they can be credited:
    1. Systmatic error may occur, e.g. zero error in burrette reading.
    2. Random errors may occur. C02 from the air may dissolve in solution, making it more acidic, affecting the accuracy of pH measured.

    Alyson, Block 6

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  20. Dear Sir,

    Hope this response finds you well. I don’t really have a good enough excuse for my delay, which is completely wrong of me. I’m really sorry and I hope this won’t happen again in the future.

    As for the paper,

    Question 2.) Paper 51 M/J 14,
    I had no difficulty with the graph. However, when it came to Question 2.)d.)ii.), I had to calculate the standard electrode potential multiple times because I kept getting a value that didn’t correspond to the estimated value of my y-intercept. I tried my luck anyway and managed to get a value that differed by 0.02V, probably due to the fact that I used a point on the graph instead of a fixed point? I also got Pb instead of Sn, but I was wondering if my value is still acceptable even if it doesn’t correspond to any values in the redox values table? That’s one of my concerns when it comes to questions like these – how do we suggest an answer if none of the data we have matches exactly what’s given? How do examiners mark those kind of questions? Regarding question 2.)g.)ii.), I managed to eliminate KCl from the list but chose K2SO4 instead. Still trying to figure out why it isn’t so, though. But I do remember you mentioning something about it in class..

    Question 1.) Paper 51 M/J 11,
    I actually did this question last year (during our “summer” break) because Puan Umi gave us a bunch of Paper 5 Reaction Kinetics question to work on over the holidays. So, I didn’t really experience much difficulty with this question. But I do remember being uncertain because I wasn’t sure of what values to suggest. But overall, it was an okay question. Would like to have some examples of experiments that can help us with this kind of question – the kind that asks you to design an experiment.

    Question 2.) Paper 51 O/N 14,
    I have always been terrible at drawing curves, and this question was no exception. My curve was really bad that I managed to get two anomalous points. I am really really really bad at drawing curves. Trying to find a way around it though. I also read question 2.) c.) wrong and suggested an indicator instead, so carelessness is still apparent. Oh and I also calculated the Ka value wrong, because I used pH 2 instead of pH 2.41. So careless! Question 2.) f.) ii.) was tricky for me, but I thought I worked it out correctly. I’m also still unsure of how this answer was obtained!

    Overall, I think the questions were challenging, but they certainly tested my knowledge. I especially hope there’s a way for us students to tackle experimentation questions because they’re pretty difficult. I mentioned this to you when I inquired about it the other day, regarding how in Physics, I was taught a format of how to answer experimental design questions, but we lacked that in Chemistry.

    Hope this suffices!

    Diyanah, Block 4.

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  21. Problems I faced :-
    M/J 14 51
    c) I was tricked into thinking that there would be an anomalous value. I also did not know how to explain why there should be no anomalous points.
    e) I could not identify what M was because I kept using 0.8+0.93 to calculate the electrode potential. It wasn’t until I read the mark scheme that i realised my mistake.

    O/N 14 51
    d) I struggled with this question as I just could not think of a way to relate pH and Ka.

    Generally, I find that I struggle most with questions involving calculations. I think that the only way to overcome this problem is through practice. Also, I am losing too many marks/spending too much time on general questions such as hazard and why my results are subject to error. Again, this could probably be mastered by doing more practice questions. Finally, there is also the issue of time constraint, which seems to be my biggest problem for all subjects and papers.

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  22. Problems I faced : –
    Regarding the paper that was given out today, I ran into a few problems.

    Q1 c) . 51/MJ/14
    The first question caught me off guard as I was thinking that there’s always an anomalous point on our curves , so I forced the answer even though the points were close to the line. I was surprised when I looked at the scheme, so this taught me to not assume an answer.
    (ii) Q51/MJ/14
    Was careless with the positive and negatives sign so I got Ferum instead of the given answer of Pb.
    (iii) Q51/MJ/14
    Comparisons must be made between all 3 salt bridges , however , I just compared between two possible answers which gave me 2 out of a possible 3 marks.
    d) 51/M/J/11
    A classic planning question , struggled to provide all the points needed by the scheme , as I didn’t tackle the question precisely. Learnt that factors of burning can be controlled using physical means on the intensity setting on the bunsen burner.
    g) 51/M/J/11
    Wasn’t sure about the improvement on the apparatus , but realised that any changes to the previous apparatus and as long as it makes sense , it will be plausible.

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  23. Sir,
    I encountered several problems while doing the papers.

    For m/j/14,
    2. C) most papers I’ve done before, well actually all of them, there have always been anomalous points. So I was sort of misled into thinking this one did have an anomalous point too.
    F) I forgot that lead precipitates and would form a precipitate with potassium sulphate as the salt bridge solution. Careless.

    For m/j 11,
    I found this paper to be particularly difficult. I was unsure of many things such as potential hazards and also method to Control the factors of heating.
    Also, most of the time I find my answers to be really different from the answers in the mark scheme especially when it comes to questions like these, when you have to improve an experiment and so on. Is there a way to improve on this?

    For o/n 14,
    1. Once again, identifying errors was a problem. Especially when the question stated that apparatus used was highly accurate. I’m at a loss at how to answer these type of
    questions.

    Thank you.
    -Dhivyaa,block 5-

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  24. Good day and sorry for the delay.

    M/J 2014/51
    – I made an error in the question asking for anomalous points. I now know that anomalous points should be obvious.
    – I didn’t have a problem with the rest of the questions but I made a careless error for the last one. I forgot that lead ions will react with sulfate ions to form a precipitate.

    M/J 2011/51
    – I faced a lot of difficulty answering this paper in detail.
    – I struggled to keep my description of the experimental procedure organized because I forgot some steps and needed to add them in later on.
    – I thought fixing the number of moles of the carbonates was unnecessary since a fixed volume of carbon dioxide would be collected regardless of how much carbon dioxide was produced. (I still don’t understand why we need to do so.)
    – I actually struggled thinking of a hazard to be considered.
    – As for the alternative apparatus/ improvement to apparatus question, I had the idea to use electrical heating for even heating but I did not put in the name of an apparatus (a hot plate).

    O/N 2014/51
    – Carelessly, I missed out a point when plotting the graph and it was the anomalous point at 12.0 cm^3. So, I lost marks for the anomalous points question too.
    – I did not know how to answer (d) nor (g).

    Overall, I lost a lot of marks from being careless and not reading the questions carefully. I didn’t struggle too much with the calculation questions. I have a problem when it comes to explanation questions though. No doubt, I need to strengthen my understanding of various topics to answer these questions properly.

    I think that’s all I have to say. Sorry again for the delay and thank you.
    -Cheow Lin (Vanessa), Block 4

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  25. Hello sir,

    1) Q C) MJ_14 I don’t really know how to explain the absence of anomalous point, but still I can’t understand the marking scheme when it says that ‘variation in points due to rounding’. If the points isn’t rounded either, will there be any anomalous point?.I guess not. For Q a) ON_14_51 i deliberately draw the graph using all the points given, while the anomalous point is at 12.00cm3. It was because i did thought weak acid should have ‘buffer region’?

    2) Q g) MJ_11_51 this was a simple question but i did have some problem with it regarding the apparatus we have/should use as answers. Can we use any other apparatus which aren’t available in a standard lab because I’m pretty sure the boiling tube and syringe is the best apparatus we can find in lab for the experiment.

    3) Q e), f)i) and f)ii) ON_14_51 I did stuck on this question but i guess the trick to it is to make a simple analysis of the graph. As half of the volume of base added is when the [A-] = [HA] so that pKa=pH . This works between weak acid and a strong base.

    I did lose marks on some questions such as the hazard in the lab and Q d) ON_14_51 as i did not read the question thoroughly. Other than those Qs, carelessness in calculations and bad techniques in writing preparation of experiments.
    I guess, the best thing to do is to keep doing past year papers and revise.
    Thank you.

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  26. Hello Sir! These are some of the few dodgy questions and difficulties that I have came across whilst doing the Papers:-

    1. M/J 2014 51
    I believe I have quite a few problems with planning/designing experiment questions. As such, some of my diagrams aren’t appropriate e.g for 1(b)(i) I used slaked lime to absorb the NO2 instead of a liquid alkali and following that, my suggestions for measurements to be taken in the subsequent questions were incorrect. Also, the techniques involved in obtaining accurate results are rather new to me and I wouldn’t have thought of implementing them at all!

    2. O/N 2014 51
    I was already struggling big time with the first question. It has completely slipped my mind that copper metal is not reactive enough to react with acid (nasty SPM stuff). Following this copper business, I was surprised that the copper has to be treated (purified?) with propanone and dried to obtain an accurate result as I was thinking that using excess acid is sufficient to extract the exact amount of copper (the copper can’t possibly still wet with acid hence increasing its weight right?). And still on question 1, for (f) how is the percentage of copper considered small (15%) and yet it’s fine for aluminum (20%). Such small discrepancies in the percentage weight should not be an issue and to say that the mass of copper obtained is too small as an alternate reason would also imply that mass of aluminum is small as well. For question 2, I find reasons to justify anomalies a little tricky as we have to dig deep(er) and CIE employs this question in all the paper 5 so that’s just wonderful.

    3. M/J 2011 51
    For a(ii) I wouldn’t have used Mr on the x-axis; I was thinking more along the line of ionic radii as it is directly related to charge density. As with previous questions, I struggled with planning of experiments and the question 1(d) was no different. I must say that’s a mighty lot of lines for a 4-mark question and not the most pleasant thing to see at first sight. Also, the marking scheme suggested the technique of ‘sucking back and removing delivery tube’ as a precautionary step. I have never heard of it much less think of it as an answer. I certainly don’t remember sucking and pulling back any tubes in my AS Chemistry Practical days. A little problem with plotting points for graph in question 2 and same old anomaly issue.

    That’s about it from me Sir!
    Desmond Chai – Block 5

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  27. Overall an ‘OK’ paper.For the first question, part c); My reason for the anomalous points would be that the points are close to the line. (which would not have earned me the mark.)

    For the second question part (d)(iii); apparently to control the heating, the distance from the Bunsen burner to the apparatus must be kept constant.

    TQ.

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  28. Hello Mr.Allan,
    Im extremely for the delay but the comments below are the thoughts I had while doing the papers given.
    1) 51/M/J/14
    – 2(c). After drawing the best fit line,there are a couple of points which i missed but they are not that far off. I was contemplating to assume that those points are anomalies

    2) 51/M/J/11
    -this paper took me the longest time to complete out of the three given. It took me awhile to scan and understand the question and the designing part did not help my timing. it must be due to lack of practice.

    3) 51/O/N/14
    -2(b) The question told us to draw a smooth curve using the plotted points. I connected all the points which made me confused after reading question. Didnt know that one of the points is an anomaly
    -2(e) Spent alot of time thinking about this particularly. The only equation that i can relate with this question is pH= pka +log [base]/[acid]. However i had difficulties in identifying the ratio of the base and acid.

    Overall sir, I do find all three questions tricky. Pretty sure however that it’ll be more doable after more exercises.

    Like

  29. Hello Sir,
    I am truly sorry for the delay. Here’s the evaluation that I have done, and also important aspects that I need to improve.

    9701/51/M/J/14 –
    d)i)I was unable to do the calculation until I realised that I have to use y=mx+c . Definitely need to familiarise myself with this kind of questions because it’s actually simple and not as hard as I might think.
    The position of 0.0 on x axis is also quite unusual because the scale extends to the negative values as well, need to be extra careful when finding c.

    9701/51/M/J/11
    d) The planning question is hard for me because I thought I have to outline all the procedures. However, it turns out that stating what exactly the questions asked is necessary and sufficient to gain marks.
    I have made a mistake by using the same mass of carbonates, which is impossible in order to get the same no of mol, because each group 2 carbonates has their own molar mass.
    Need to consider the relevant of the question asking gas volume first, only then can work out the mass of each carbonate.
    g) I’m worried about this part, because none of the suggestion seems familiar to me e.g regulated heating device. I think I need to do more questions regarding alternative apparatus.

    9701/51/O/N/14
    d) I didn’t read and understand the statement given properly, and so did not realised that only 15 cm3 of NaOH has been added to the acid, which is not the neutralisation because 30cm3 is required instead.
    – The calculation part is hard , because I thought I have to find [HA] in order to find Ka. However since only H+ react with OH- , so when 15 cm3 NaOH is added (half the volume required for neutralisation) it won’t affect [HA], thus [HA]=[A-] , so they cancel out each other This is from what I understand, please correct me if I am wrong 🙂

    That’s all from me. Thank you Sir.
    Nurizati

    Like

  30. Hi Sir, really sorry for the delay!

    Here are a few problems I encountered while doing the questions:

    M/J 2011 Variant 51
    1. For the graph sketch in question 1(a(i), I labelled the x-axis with charge density instead of molecular mass. So instead of drawing a straight line with negative gradient, I drew a straight line with positive gradient. I also wrote charge density as the dependent variable when it should have been molecular mass.

    2. For question 1(d)(iii), I thought constant intensity of heating would be sufficient for the answer, but 2 points were awarded for this question, which are for “same setting of Bunsen burner” and “same distance from reaction vessel” respectively.

    M/J 2014 Variant 51
    1. For question 1(d)(ii), I have written “make sure all connections are air tight and smear joints with vaseline” which was incorrect. I later understood that the expansion of gases will affect the volume measured so it is important to allow the apparatus to cool.

    2. For question 2(c), it was quite novel to find no anomalous points in the graph and to find a reason for that, so I was quite unsure of what to put down.

    O/N 2014 Variant 51
    1. For question 2(e), I was quite unsure of how to find the concentration of HA to calculate the acid dissociation constant. As the answer to this question would affect my subsequent ones, I was stuck at it for quite some time trying to think of the right method. When I checked the mark scheme, I found that the explanations for the allocation of marks were rather vague.

    That’s all from me. Thanks sir!

    Pei Rong (Block 4)

    Like

  31. Salam sir.
    I encountered a few problems while answering.
    One of them is, drawing best fit line and finding anomalous points. This problem occured when I answered M/J 2014. Turned out, there wasn’t any anomalous point stated in the ms, when i was so determined to find one. The next paper, M/J 2011, the only problem i had the last part regarding the improvement of apparatus. I had a pretty difficult time with the calculations in the last paper particularly part e. To answer that question I realized that we have to know that pKa equals the pH at half neutralisation point.
    I am aware now of what i did wrong and where do i have to improve. I believe that answering paper 5 won’t be as hard, if a lot of practices were done.

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