A black sulphide X from group II (precipitated with $\mathrm{H}{}_2\mathrm{S}$ in 0.3 M $\mathrm{HCl}$) does not dissolve in yellow ammonium sulphide but dissolves in hot dilute $\mathrm{HNO}{}_3$. The nitrate solution gives no precipitate with dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$, and a white precipitate with excess $\mathrm{NH}{}_4\mathrm{OH}$ whose filtrate is colourless. The white precipitate gives a black deposit with alkaline sodium stannite. How many moles of stannite $[\mathrm{Sn}(\mathrm{OH}){}_4]{}^{2-}$ are oxidised per mole of the metal ion being reduced?

1. AX is $\mathrm{Bi}{}_2\mathrm{S}{}_3$; 3 mol of stannite per mol of $\mathrm{Bi}{}^{3+}$
2. BX is $\mathrm{PbS}$; 1.5 mol of stannite per mol of $\mathrm{Pb}{}^{2+}$
3. CX is $\mathrm{CuS}$; 1 mol of stannite per mol of $\mathrm{Cu}{}^{2+}$
4. DX is $\mathrm{Bi}{}_2\mathrm{S}{}_3$; 1.5 mol of stannite per mol of $\mathrm{Bi}{}^{3+}$