This set of questions stood out as something that we can
work on. You all knew hat pH was and how it related to strength and
concentration but the way that these concepts are expressed is crucial – you
have to be very careful with terminology (eg weak & strong) and you have to
clearly make the link between strength, concentration [H+] and pH.
WARNING – AVOID THE TRAP
we really have to stop falling for the tricks where Q ask about increasing or
decreasing pH. HIGHER [H+] = LOWER pH
2001
Q20b) You
need to correctly use the terms strong and weak, You need to link them to %I
and thus the final hydrogen ion CONCENTRATION and thus to pH via the formula. ‘Even though both acids are at the same
concentration (0.1 mol/L) , HCl is a strong acid with 100% ionisation and thus
will have a [H+] of 0.1 mol/L. Citric acid is weak and thus has a %I < 100
& will have [H+] of less than
0.1mol/L. Thus the pH of the hydrochloric acid is lower than citric acid as pH
= -log[H+] (ie pH HCl = -log[0.1] = 1) and thus as [H+] increases, pH
decreases’ Note you could figure out the citric and HCl %I form the data –
this is a GREAT way to answer these questions and reinforce the concept of
strng and weak. But it is also something to watch for because according to the
data in this question %I for citric is 25% which means that for THIS QUESTION
saying the %I of citric is 8% is technically wrong.
2002
Q22b &c) for b) you needed to know that
sulfuric acid was diprotic, thus can produce a higher [H+] then HCl when at the
same concentration, so sulfuric acids pH will be lower. For c) ditto Q20b)
2001e above, without the calculations.
2003
Q8) We have been over the diprotic nature of
sulfuric acid SO many times…
2004
Q7) WARNING – AVOID THE TRAP
we really have to stop falling for the tricks where Q ask about increasing or
decreasing pH. HIGHER [H+] = LOWER pH
Q24a) This was dilution so use the dilution
equation. In addition the #SF in the ‘normal’ numbers in the data is used to
calculate the # decimal places in a pH answer (& the #decimal places in pH
data = the #SF you need when calculating the answer)
Q24b) If
you are given two chemicals and asked about their uses you should give a
different use for each. Also Citric acid is NOT ascorbic acid (vitamin C). Citrus
fruits have both these acids in them but it does not mean that they are the
same thing. Just like, Pepsi has water and sugar and caffeine in it but they
are different chemicals.
Q24c) You have to give the pH equation. You have to
say that higher [H+] = lower pH then you have to say why the [H+] – ie hydrogen
ion CONCENTRATION - is different in each and thus why the pH is different in
each. Many fell into the trap of talking about the ‘amount’ or moles of H+ in
solution – THAT DOES NOT MATTER. pH depends on hydrogen ion CONCENTRATION. Also
– NEVER talk about things being ‘more’ or ‘most’ or ‘less’ acidic. This means
nothing in Chemistry and reflects badly on you as a Chemistry student. If you
mean ‘stronger’ say ‘stronger’. If you mean ‘more concentrated’ say ‘more concentrated’. Finally you need to
ay that HCl was strong a t100% ionised and that citric and acetic were weak but
had different [H+] as citric is 8% ionised and acetic 1% ionised(giving citric
a higher hydrogen ion concentration and a lower pH than acetic).
2005
Q8) We
have been over the diprotic nature of sulfuric acid SO many times…
2007
Q10) “...must be added…” I’ve thrown this
trick at you a few times. It is cruel but you need to be aware and looking for
it in ALL dilution questions.
Q21a&b) We have been over the diprotic nature of
sulfuric acid SO many times…
Q21c) The same inexplicable numerical error showed
up in approximately half of the papers. Again – work together to discuss
solutions to the questions but go and work independently. IF you are just
copying answers without thinking: a) it is not helping b) you are not learning, c) you are not showing
the person you are copying from respect (because if you were thinking you might
have found their mistake and helped them) & d) it means that I can’t fully
diagnose problems you are having.
2008
Q14) This
was basically identical to 2006 Q17b and 2012 Q28 but we did well in those and
poorly in this. The main error was using the moles of hydrogen ions rather than
the concentration in the pH calculation. A silly, avoidable, mistake.
2010
Q21) You need to distinguish yourself from the
crowd so if you are given data USE IT! Just like 2001 Q20b – you have the data
to calculate the %I so do so. Basically if you ever get a compare the pH
question and you can calculate %I then do it. (remember in our first assessment
the rule that if you were provided with a stimulus you had to use it in your
answer – this is the same – if provided with numbers and you can do a
calculation to support your answer then do the calculation)
2012
Q11) WARNING – AVOID THE TRAP
we really have to stop falling for the tricks where Q ask about increasing or
decreasing pH. HIGHER [H+] = LOWER pH. You shoud know the different %ionisation of hydrochloric, acetic an citric acids.
Q18) 1) Weak acids are in EQUILIBRIUM with
their conjugate base, water and hydronium. Changing any of these chemicals will
shift the equilibrium. Eg CH3COOH (aq) + H2O(l) ↔ CH3COO-(aq) + H3O+(aq) Diluting is adding water, thus by LCP the
reaction will shift to remove water, thus reducing the acid conc and increasing
the conc of conjugate and hydronium – thus raising the %I.
2) WARNING – AVOID THE TRAP
we really have to stop falling for the tricks where Q ask about increasing or
decreasing pH. HIGHER [H+] = LOWER pH
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