Question

At 300 K, vapour pressure of pure liquid A is 70 mm Hg. It forms an ideal solution with liquid B. Mole fraction of B = 0.2 and total vapour pressure of solution = 84 mm Hg. What is vapour pressure (in mm) of pure B?

• A. 140
• B. 70
• C. 280
• D. 560

Hint:

Use Raoult’s law for ideal solutions: $p_{total} = x_A p_A^\circ + x_B p_B^\circ$

Answer 1

The Correct Option is A

$x_A = 0.8$, $x_B = 0.2$
$p_A^\circ = 70$, $p_B^\circ = ?$
$p_{total} = x_A p_A^\circ + x_B p_B^\circ = 84$
$84 = 0.8 \times 70 + 0.2 \times p_B^\circ$
$84 = 56 + 0.2p_B^\circ \Rightarrow 0.2p_B^\circ = 28 \Rightarrow p_B^\circ = 140$ mm Hg