Question

The vapour pressure of benzene at a certain temperature is 40 mm of Hg. A non-vapour and non-electrolyte solid weighting 2.175 g added to 39 g of benzene. The vapour pressure of the solution is 600 mm of Hg. What is the molecular mass of solid substance?

• A. 65.25
• B. 59.6
• C. 59
• D. 79.8

Answer 1

Answer: (A)

65.25

vapour pressure of pure benzene at certain ttemperature be $P{A}^0=640mmHg$

Let number of mole of electrolute = $n$

Number of mole of benzene = $\frac{39}{78}=0.5$

Mole fraction of benzene = $x_A=\frac{0.5}{0.5+n}$

$p=p_A.x_A$

$600=640\frac{0.5}{0.5+n}$

$30+60n=32$

$60n=2$

$n=\frac{2}{60}$

$\frac{2.175}{M}=\frac{2}{60}$

$M=\frac{2.175\times 60}{2}=65.25g$