2022 O Level Chem P1 Q22 — Qualitative Analysis / Salts Solution

Answer: D. Track the ions step by step.

Step 1 — warming \((\text{NH}_4)_2\text{SO}_4\) with excess NaOH destroys the ammonium: \(\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}\). The \(\text{NH}_3\) escapes as gas. The solution now contains \(\text{Na}^+\), excess \(\text{OH}^-\) and \(\text{SO}_4^{2-}\).

Step 2 — adding dilute HCl neutralises the excess NaOH: \(\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}\). The solution now contains \(\text{Na}^+\), \(\text{Cl}^-\) and \(\text{SO}_4^{2-}\).

Step 3 — adding aqueous \(\text{BaCl}_2\) precipitates \(\text{BaSO}_4\) (white). After filtering, the filtrate contains \(\text{Na}^+\) and \(\text{Cl}^-\) (plus any excess \(\text{Ba}^{2+}\)).

Step 4 — evaporating gives the white solid sodium chloride. The other options either ignore that the ammonium was destroyed (A, B), or refer to the precipitate that was filtered off (C).