Mohr’s salt is a pale green crystalline solid that dissolves in water. It contains two cations, one of which is $\text{Fe}^{2+}$, and one anion, which is $\text{SO}_4^{2-}$. The second cation was identified by heating solid Mohr’s salt with solid sodium hydroxide, when a colourless gas was given off. This gas dissolved easily in water to produce an alkaline solution. A grey-green solid residue was also produced, and this residue did not dissolve in water. What are the identities of the gas and the solid residue?
- Agas: $\text{NH}_3$, residue: $\text{Fe(OH)}_2$
- Bgas: $\text{NH}_3$, residue: $\text{Na}_2\text{SO}_4$
- Cgas: $\text{SO}_2$, residue: $\text{Fe(OH)}_2$
- Dgas: $\text{SO}_2$, residue: $\text{Na}_2\text{SO}_4$