In (d), candidates could not describe how the conductance of a molar solution of ammonia compares to that of sodium hydroxide solution.
The expected response from candidates was as described below:
- pour some quantity of each solution into a separate beaker.
- insert electrodes into the solutions and close the circuit.
- take the readings on the ammeter.
- rinse the electrodes properly with distilled water and dip them into the second solution. Close the circuit and take the reading on the ammeter again.
- the solution with the higher reading is NaOH(aq) while the one with lower reading is ammonia.
OR
NaOH solution is a strong electrolyte and completely ionizes to produce a lot of ions. NH3 solution is a weak electrolyte and produces very few ions. The ions produced by NaOH far outnumber those produced by NH3 solution. Hence NaOH solution conducts more than NH3 solution.
In (e)(ii), candidates correctly stated esterification as the reaction illustrated by the equation. However, some of them lost the mark in (e)(ii) because they confused neutralization for hydrolysis.
In (f), candidates determined the volume of the residual gas as follows:
2H2(g) + O2(g) → 2H2O(1)
from the equation
2 vols of H2 reacts with 1 vol of O2/20cm3 of H2(s) reacts with 10 cm3 of O2(g)
residual gas = (15 – 10)cm3 = 5cm3.