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

Hint: Vapour pressure is defined as a measure of the tendency of a substance to change its state into the vapour or gaseous state, and the vapour pressure is dependent upon temperature and it increases with temperature. Vapour pressure can be used to define the boiling point which states that the value of temperature at which the vapour pressure of a liquid surface becomes equal to the pressure exerted by its surroundings.

Complete step by step answer:
So According to the question
Given data,
Vapour pressure of P = 80 torr
Vapour pressure of Q = 60 torr
Number of moles of P = 3
Number of moles of Q = 2
To proceed with the question let us first find the mole fraction of the given liquids P and Q.
The mole fraction can be found out by dividing the given number of moles to total number of moles.
Therefore, Mole fraction of P = $\dfrac{3}{{3 + 2}} = \dfrac{3}{5}$​
And, Mole fraction of Q = $\dfrac{2}{{3 + 2}} = \dfrac{2}{5}$
The total vapour pressure is given by = (Mole fraction of P × Vapour pressure of P) + (Mole fraction of Q × Vapour pressure of Q)
$ \Rightarrow (\dfrac{3}{5} \times 80 + \dfrac{2}{5} \times 60) = 48 + 24 = 72$
Therefore the total vapour pressure of the mixture is 72 torr

Hence, the correct option is D.

Note:
When heating of liquid is done, its molecules obtain enough amount of kinetic energy to overcome the forces that stable them in the liquid and they escape into the vapour phase. By undergoing this process all the molecules change their state and the population of vapour above the liquid phase is formed that produces the pressure called vapour pressure.