Question

Moles of $K_2SO_4$ to be dissolved in 12 moles of water to lower its vapour pressure by 10 mm Hgat a temperature at which vapour pressure of pure water is 50 mmHg is

• A. 3 mol
• B. 2 mol
• C. 1 mol
• D. 0.5 mol

Answer 1

Answer: (C)

1 mol

Answer 2

$\frac{\Delta p}{p} = \frac{in_B}{in_B + n_A}$ (Please note this step) Here $i$ = 3 $\frac{10}{50} = \frac{3n_B}{3n_B + 12}$ $3n_B + 12 = 15 \, n_B$ $12 \, n_B = 12$ $n_B = 1$