A certain metal sulphide ${\mathrm{MS}}_2$ is used extensively as a high-temperature lubricant. If ${\mathrm{MS}}_2$ is $40.00\%$ by mass sulphur, atomic mass of $\mathrm{M}$ is

$\%$ weight of sulphur in ${\mathrm{MS}}_2=\frac{\mathrm{Weight} \mathrm{of} \mathrm{sulphur}}{\mathrm{weight} \mathrm{of} \mathrm{compound}}\times 100$

$$\begin{gathered} \Rightarrow 40=\frac{64}{64+\mathrm{x}}\times 100 (\mathrm{Where}, \mathrm{x} \mathrm{is} \mathrm{the} \mathrm{atomic} \mathrm{mass} \mathrm{of} \mathrm{M}) \\ \Rightarrow 2560+40\mathrm{x}=6400 \\ \Rightarrow 40\mathrm{x}=3840 \\ \Rightarrow \mathrm{x}=96 \end{gathered}$$

Hence, atomic mass of $M$ is 96 .