Search results

Question 26 Many students struggled here because they accepted what the maths said without considering the chemistry. The average score was 1.458 / 3 in this question. A common mistake was to take the pH = 10.54 solution and do a 1-in-10 dilution to conclude that [H+] = 10-10.54 was diluted to...

I have uploaded for each student who did the BoS 2021 Chemistry trial your marked paper into the forum where you submitted the papers. It is my understanding that notifications that they are uploaded aren't coming through, so if anyone can't find there paper, send me a message. The comments...

Question 25 Part (a) averaged 0.286 / 1 amongst those who attempted the question Part (b) averaged 1.571 / 2 amongst those who attempted the question Part (c) averaged 0.417 / 1 amongst those who attempted the question Compounds A and B are isomers of C3H7Br, which means they can only be the...

Question 24 Part (a) averaged 0.708 / 1 Part (b) averaged 0.625 / 1 Part (c) averaged 1.375 / 2 Part (d) averaged 1 / 2 The system involved was 2 A (g) + C (g) <---in equilibrium---> 2 B (g). The possible mistakes were: not recognising that "synthesis" meant one product on the RHS, which...

Question 23 This question was handled well. Every response scored 2 or 3 out of 3, and the average was exactly 2.5. There was no particular answer sought or points that needed to be raised, but there did need to be: both advantages and disadvantages considered some discussion - a list of five...

Question 22 Part (a) averaged 0.917 / 2 Part (b) averaged 1.042 / 2 For 2 marks in part (a), it was necessary to note cover (n some way): An indicator has (at least) two forms, existing in equilibrium (such as HInd + H2O <---in equilibrium---> H3O+ + Ind-), and that these forms have different...

Question 21 This question proved to be quite challenging. Part (a) averaged 0.875 / 1 Part (b) averaged 1.458 / 3 I had two answers in mind in relation to part (a) - that the BP gap for water and 2-propyl propanoate could be a problem, and that 2-propanol having the lowest of the BPs meant...

The marked papers for those who did the exam, which include individual comments, will be posted to the forum where the exam was done as soon as I know that they can be accessed. I'm also going to make some general comments today, and will answer specific questions.

Yes. Without the basic salt, the pH of the acetic acid alone would be somewhere near 3 (concentration depending, of course).

Here is a box-and-whisker plot of the results: On the left, in blue, is the plot based on raw marks / 100. We see a median slightly above the mean, only one outlier. On the left, in orange, are percentages of marks obtained from questions attempted. So, as mentioned above, a candidate who...

First, let me start with an apology that this process has taken much longer than expected. Second, here are the distributions of answers for the MCQ: The most difficult questions, based on the rate at which the question was answered correctly, were questions 5, 14, 11, and 2. The easiest...

So long as you are aware of the limitations in your understand of modules 5 and 6 and make sure they are addressed, I don't see a problem.

You can indeed prepare a buffer solution with ethanoic acid and sodium hydroxide. As you say, it would be an effective buffer when ethanoate ions and ethanoic acid are present in equimolar quantities. The problem is that the pH of the buffer would be 4.76, and thus it would be an acidic...

Yes, Y was 2,2-dimethylbutanoic acid, though the question was meant to ask for the name of Z. The oxidant was the diamminesilver(I) cation, [Ag(NH3)2]+. It was formed, as stated in the question, by reaction of silver(I) nitrate with sodium hydroxide to form silver oxide: 2 AgNO3 (aq) + 2 NaOH...

I meant in the context of purifying an ester. The mixture after reflux includes unreacted carboxylic acid and sulfuric acid. Neutralisation removes the sulfuric acid catalyst, meaning that any subsequent movement in the position of equilibrium will be slower. Neutralising the unreacted...

This is not an unusual sequence, but it is important that aspects of modules 5 and 6 that are applied in module 7 be picked up at an appropriate point later on.

If by "merit" you mean marks in an exam, marking can only be done based on what is written in responses, not on what an examiner might think the student understands, etc. So, a response that answers the question will always gain credit. Verbatim memorisation becomes much less necessary...

Yes, I believe that exams where a strong performance is possible without use of the brain are not particularly worthwhile. Those who perform well should be demonstrating the ability to think like a scientist, to apply knowledge to solve unfamiliar problems, and to reason sensibly even if a...

Soaps can also be salts of sulfonic acids so the head group need not necessarily be a carboxylate. Something like sodium dodecyl sulfate is common in shampoo and works just like a soap to clean hair (CH3(CH2)11SO3Na).

1H NMR gives information on: The number of hydrogen environments in a molecule The ratio of hydrogens in the environments (from integration / intensity) The shielding / deshielding effects from moieties near each environment (notably electronegative atoms / electron density) via chemical shifts...