The vapour pressure of a solution having $2.0\,\mathrm{g}$ of solute $X$ (molar atomic mass $=32\,\mathrm{g\,mol^{-1}}$) in $100\,\mathrm{g}$ of $\mathrm{CS_{2}}$ (vapour pressure $=854\,\mathrm{torr}$) is $848.9\,\mathrm{torr}$. The molecular formula of the solute is