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 $60$ respectively?

• A. $80$ torr
• B. $140$ torr
• C. $720$ torr
• D. $70$ torr

Answer 1

The correct option is D $70$ torr

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 $Q$)
$=(\frac{3}{5}\times 80)+(\frac{2}{5}\times 60)=48+24=72torr$