If the formula of the nitride of a metal X is XN, what is the formula of:

Its sulphate.

• From the formula of metal nitride,$\mathrm{XN}$, we find that charge of $\mathrm{X}$ is +3 as the charge of nitride is -3.
• Both balance each other to form the formula$\mathrm{XN}$.
• For example, $\mathrm{AlN}$, etc.
• Now, we know that the charge of sulfate (${{\mathrm{SO}}_4}^{2-}$) is -2 and the charge on metal $\mathrm{X}$ is +3.

By applying the criss-cross method, the formula will be ${\mathrm{M}}_2{\left({\mathrm{SO}}_4\right)}_3$

$$\begin{gathered} {{\text{Symbol X SO}}_4}^{2-} \\ \text{Valency 3↗ ↖ 2} \end{gathered}$$

Therefore, the formula of metal sulfate is ${\mathrm{M}}_2{\left({\mathrm{SO}}_4\right)}_3$