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 mole of P and 2 mole of Q would be:

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

Answer 1

The correct option is C 72 torr

The formula for calculating the vapour pressure of a solution is given by:

$P_T=P_P^0{\chi }_P{+P}_Q^0{\chi }_Q$

Where $P_T$ = Vapour pressure of the solution

$P_P$ = Vapour pressure of liquid 'P' in pure state

$P_Q$ = Vapour pressure of liquid 'Q' in pure state

${\chi }_P$ = Mole fraction of liquid P in the solution

${\chi }_Q$ = Mole fraction of liquid Q in the solution

In this question,

$P_P$ = 80

$P_Q$ = 60

moles of P=3 and Q=2 int the solution, So total moles = 5

${\chi }_P$ = 0.6

${\chi }_Q$ = 0.4

Hence $P_T$ = 80 x 0.6+ 60 x 0.4 = 72.