Answer: B. Test 1 — warming with aqueous NaOH releases a gas that turns damp red litmus paper blue. This is the test for ammonia, \(\text{NH}_3\): \(\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}\). The cation present must be \(\text{NH}_4^+\).
Test 2 — adding dilute \(\text{HNO}_3\) (which removes any carbonate interference), then aqueous barium nitrate \(\text{Ba(NO}_3\text{)}_2\), gives a thick white precipitate. This is the standard test for sulfate ions: \(\text{Ba}^{2+} + \text{SO}_4^{2-} \rightarrow \text{BaSO}_4\)(s) — a white precipitate of barium sulfate confirms sulfate is present.
So the powder contains \(\text{NH}_4^+\) and \(\text{SO}_4^{2-}\) — option B (ammonium ions and sulfate ions). A introduces chloride which was never tested for. C drops ammonium, contradicting test 1. D contains neither of the confirmed ions.
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