Choice C is correct because:
- Mohr's salt is a double salt containing Fe2+ and NH4+ ions. The formula of Mohr's salt is (NH4)2Fe(SO4)2.6H2O.
- When Mohr's salt is heated with sodium hydroxide, ammonia gas (NH3) is evolved. This is because the ammonium ion (NH4+) reacts with the hydroxide ion (OH−) to produce ammonia gas and water. The reaction is: NH4+(aq)+OH−(aq)→NH3(g)+H2O(l)
- The gas readily dissolves in water to give an alkaline solution because ammonia is a weak base. NH3(g)+H2O(l)⇌NH4+(aq)+OH−(aq)
- The iron(II) ions (Fe2+) react with hydroxide ions (OH−) to form iron(II) hydroxide (Fe(OH)2), which is a grey-green precipitate and is insoluble in water. The reaction is: Fe2+(aq)+2OH−(aq)→Fe(OH)2(s)
- Therefore, the gas evolved is ammonia (NH3) and the solid residue is iron(II) hydroxide (Fe(OH)2).
Choice A is incorrect because:
- Hydrogen gas (H2) would not be evolved when Mohr's salt is heated with sodium hydroxide. The reaction between ammonium ions and hydroxide ions produces ammonia gas, not hydrogen gas. Also, FeSO4 would be soluble, not insoluble.
Choice B is incorrect because:
- While ammonia gas (NH3) is indeed evolved, the solid residue is not sodium sulfate (Na2SO4). Sodium sulfate is soluble in water, but the question states that the residue is insoluble. The iron(II) ions react with hydroxide ions to form insoluble iron(II) hydroxide.
Choice D is incorrect because:
- Sulfur dioxide (SO2) would not be evolved when Mohr's salt is heated with sodium hydroxide. The reaction involves the ammonium and iron(II) ions, not the sulfate ions. While iron(II) hydroxide (Fe(OH)2) is the correct solid residue, the gas is incorrect.

Group 2 | 2009 - M/J - 9701/1 | Q18

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Group 2 | 2009 - M/J - 9701/1 | Q18

Question (18/40)

Select your answer: