Mohr’s salt is a pale green, crystalline substance that dissolves in water. It contains two cations, one of which is $\text{Fe}^{2+}$, together with one anion, $\text{SO}_4^{2-}$. The identity of the second cation was found by warming Mohr’s salt with aqueous sodium hydroxide. A colourless gas was given off, and it dissolved readily in water to produce an alkaline solution. A green precipitate also appeared. What are the identities of the gas and the precipitate?
- A$\text{NH}_3$ and $\text{Fe(OH)}_2$
- B$\text{NH}_3$ and $\text{Na}_2\text{SO}_4$
- C$\text{SO}_2$ and $\text{Fe(OH)}_2$
- D$\text{SO}_2$ and $\text{Na}_2\text{SO}_4$