On adding I g arsenic to 80 g benzene, the freezing point of benzene is lowered by 0.19^oC. The formula of arsenic is $\left({\mathrm{K}}_{\mathrm{f}}\right)=50.8 \mathrm{K} \mathrm{kg} {\mathrm{mol}}^{-1})$.

$\begin{array}{l}{\mathrm{\Delta T}}_{\mathrm{f}}=\frac{1000{\mathrm{K}}_{\mathrm{f}}\mathrm{w}}{\mathrm{mW}}\\ {\mathrm{\Delta T}}_{\mathrm{f}}=0{.19}^{\circ }\mathrm{C};{\mathrm{K}}_{\mathrm{f}}=5.08\mathrm{Kkg}{\mathrm{mol}}^{-1},\mathrm{w}=1\mathrm{g};\mathrm{W}=80\mathrm{g}\\ \mathrm{m}=\frac{1000{\mathrm{K}}_{\mathrm{f}}\mathrm{w}}{{\mathrm{\Delta T}}_{\mathrm{f}}\mathrm{W}}=\frac{1000\times 5.08\times 1}{0.19\times 80}=334.21\end{array}$
Atomic weight of As - 74.92
Hence, member of atoms $=\frac{334.21}{74.92}\approx 4$
Hence, the formula of arsenic is ${\mathrm{As}}_4$.