The vapour pressure of Benzene is 200 mm. The lowering of vapour pressure of 0.2 molal solution is (in mm)

 Vapour pressure of Benzene = p⁰ =200 mm

Molality = 0.2 molal

Let lowering of vapour pressure = $\mathrm{\Delta p}$

Molecular mass of benzene (C₆H₆) = M = 78

By using the formula:

$\frac{\mathrm{\Delta p}}{\mathrm{p}}=\frac{\mathrm{m}\times \mathrm{M}}{1000}$

$\mathrm{\Delta p}=\frac{\mathrm{m}\times \mathrm{M}\times {\mathrm{p}}^0}{1000}=\frac{0.2\times 78\times 200}{1000}=3.07 \mathrm{mm}$

Hence, the correct option is C) 3.07mm.