Question

The vapour pressure of pure liquid A is 10 torr and at the same temperature when 1 g of B solid is dissolved in 20 g of A, its vapour pressure is reduced to 9.0 torr. If the molecular mass of A is 200 amu, then the molecular mass of B is

• A. 100 amu
• B. 90 am
• C. 75 amu
• D. 120 amu

Answer 1

Answer: (B)

$\frac{10-9}{10}=\frac{\frac{1}{x}}{\frac{1}{x}+\frac{20}{200}}$
$\frac{1}{10}=\frac{\frac{1}{x}}{\frac{1}{x}+\frac{1}{10}}=\frac{10}{x+10}$
$\Rightarrow x=90$