Let mass of solution $=100\,\mathrm{g}$ $\therefore\;\;$ Mass of benzene in the solution $=30\,\mathrm{g}$ $\therefore\;\;$ Mass of carbon tetrachloride $=100-30\,\mathrm{g}=70\,\mathrm{g}$ Molar mass of benzene $(\mathrm{C}_6\mathrm{H}_6)=78\,\mathrm{g}\,\mathrm{mol}^{-1}$ $\therefore\;$ No. of moles of benzene $\frac{\text{Mass}}{\text{Molar mass}}$ $\frac{30\,\mathrm{g}}{78\,\mathrm{g}\,\mathrm{mol}^{-1}}=0.385$ $\therefore\;$ No. of moles of benzene $\frac{\text{Mass}}{\text{Molar mass}}$ $\frac{70\,\mathrm{g}}{154\,\mathrm{g}\,\mathrm{mol}^{-1}}=0.385$ Total number of moles of solution $=0.385+0.455=0.84$ Mole fraction of benzene $\frac{\text{Moles of benzene}}{\text{Total moles in the solution}}$ $\frac{0.385}{0.84}=0.458$ ∴ Mole fraction of $\mathrm{CCl}_4 = 1 - 0.458 = 0.542$