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}\,\text{ }\!\!\times\!\!\text{ }\,\text{V}\text{.P}\text{.} $ of P + mole fraction of $
Q\,\,\text{ }\!\!\times\!\!\text{ }\,\text{V}\text{.P}\text{.} $
of Q $ =\left( \frac{3}{5}\times 80+\frac{2}{5}\times 60 \right)=48+24 $
$ =72\,\text{Torr} $