Question

The vapour pressure of two liquids $'P'$ and $'Q'$ are $80$ and $60$ torr respectively. The total vapour pressure of solution obtained by mixing $3$ mole of $P$ and $2$ mol of $Q$ would be

• A. 140 torr
• B. 20 torr
• C. 68 torr
• D. 72 torr

Answer 1

Answer: (D)

72 torr

Answer 2

Mole fraction of $P=\frac{3}{3+2}=\frac{3}{5}$ Mole fraction of $Q=\frac{2}{3+2}=\frac{2}{5}$ Hence, total vapour pressure $=$ (Mole fraction of $P \times$ Vapour pressure of $P)+($ mole fraction of $Q \times$ Vapour pressure of $Q)$ $=\left(\frac{3}{5} \times 80+\frac{2}{5} \times 60\right)=48+24$ $=72$ torr