Question

$3$ moles of $P$ and $2$ moles of $Q$ are mixed, what will be their total vapour pressure in the solution if their partial vapour pressures are $80$ and $60\, torr$ respectively?

• A. $80\, torr$
• B. $140\, torr$
• C. $72\, torr$
• D. $70\, torr$

Answer 1

Answer: (C)

Mole fraction of $P = \frac{3}{3 + 2} = 0.6$
Mole fraction of $Q = \frac{2}{3+2} =0.4$
$P_{total} = P_{p} +P_{Q} = P^{\circ}_{p}x_{p} + P^{\circ}_{Q}x_{Q}$
$= 80 \times 0.6 + 60 \times 0.4 = 72\, torr$