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 moles of P and 2 moles of Q would be

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

Answer 1

Answer: (D)

Mole fraction of $P=\frac{3}{3+2}=\frac{3}{5}$
Mole fraction of Q $Q=\frac{2}{3+2}=\frac{2}{5}$
Hence, total vapour pressure = Mole fraction of
$\text{P}\times \text{V}\text{.P}\text{.}$ of P + mole fraction of $<br/>Q\times \text{V}\text{.P}\text{.}$
of Q $=\left(\frac{3}{5}\times 80+\frac{2}{5}\times 60\right)=48+24$
$=72\text{Torr}$