Question

Two completely miscible liquids A and B form an ideal solution. What is the vapour pressure of a solution containing 2 moles of A and 3 moles of B at 300 K?
Given: At 300 K, Vapour pressure of pure liquid A ($P_A^0$) = 100 torr
Vapour pressure of pure liquid B ($P_B^0$) = 300 torr

• A. 200 torr
• B. 140 torr
• C. 180 torr
• D. None of these

Answer 1

The correct option is D None of these

According to Raoult's law, the total vapour pressure of solution is given by
$P$ $=$ $P_A^0$${\chi }_A$ $+$ $P_B^0$${\chi }_B$
Given, $P_A^0$ = 100 torr
$P_B^0$ = 300 torr
Number of moles of A = 2
Number of moles of B = 3
Total number of moles of A and B $=2+3=5$
mole fraction of A, ${\chi }_A=\frac{molesofA}{totalmoles}=\frac{2}{5}$
similarly mole fraction of B, ${\chi }_B=\frac{3}{5}$

$P$$=$$100$($\frac{2}{5}$) $+$ $300$($\frac{3}{5}$)
$P$$=$$220torr$