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

The correct option is C $72$ torr

We know that :

$P_{total}=p_P+p_Q=p_P^0{x}_P+p_Q^0{x}_Q$

where $p_P^0$ and $p_Q^0$ are partial pressures of pure substances.

$x_P$ and $x_Q$ are mole fractions.

$x_P=\frac{3}{5}=0.6$

$x_Q=\frac{2}{5}=0.4$

hence

$P_{total}=p_P^0{x}_P+p_Q^0{x}_Q=80\times 0.6+60\times 0.4=72torr$