2007 HCI Prelims Paper 2 FAQs

Explanation for Q1c
Hydromagnesite decomposes to give MgO, CO2 and H2O. MgO dissolves in excess HCl(aq) to give MgCl2 and H2O. Excess HCl(aq) added is neutralised using NaOH(aq).

Explanation for Q3a(iii)
The buffer in blood has a large reservoir of H2CO3 (weak acid) and HCO3- (conjugate base of weak acid).
When H+ is added, HCO3- reacts with all the excess H+ and converts them to H2CO3.
Thus, your task is to find the amounts of H2CO3 and HCO3- before and after the reaction. Then use pH = pKa + lg{[HCO3-]/[H2CO3]}. Note that [HCO3-] = 20[H2CO3] from 3a(ii).

Explanation for Q4b(iii)
A colourless complex is formed. [Ag(S2O3)2]3-.
Note: Ag+ also forms colourless [Ag(CN)2]- complex in excess CN-.

Explanation for Q5d(i)
When [phospholipid] is high, the rate is constant because there are not enough enzyme molecules to make the reaction go faster (enzyme = catalyst, i.e. small amt used).

Explanation for Q6a
HCOOH and (COOH)2 are easily oxidised to CO2 by strong oxidising agents.

Explanation for Q6d(i)
CH3O-Na+ is a strong base. CH3O- is like the HO- (hydroxide) ion. Thus, one of the products of the acid-base reaction is methanol, CH3O--H (HO--H if hydroxide is used).

Explanation for Q6d(ii)
RCOO- is charged and is nucleophilic. CH3I has a slightly electron deficient C atom, so CH3I will undergo (SN2) nucleophilic substitution.