Question

Equal moles of liquid P and Q are mixed . what is the ratio of their moles in the vapour phase ? Given that P p° = 2 Pq°

Answer 1

Given:

No of moles of P = No of moles of Q

P°p = 2 P°q

To Find:

Ratio of moles of P and Q in vapour phase

Calculation:

- Since equal moles of P and Q are mixed

⇒ Mole fraction of P = Mole fraction of Q = x = 1/2

- Pp = P°p × 1/2 = 2 × P°q × 1/2 = P°q

- Pq = P°q × 1/2 = P°q/2

- In vapour phase, let the total pressure be P

P = Pp + Pq

⇒ P = P°q + P°q/2

⇒ P = 3P°q/2

- Mole fraction of P in vapour phase = Partial pressure of P / Total pressure

⇒ y1 = Pp / P = P°q / (3P°q/2)

⇒ y1 = 2/3

- Mole fraction of Q in vapour phase = Partial pressure of Q / Total pressure

⇒ y2 = Pq / P = (P°q/2) / (3P°q/2)

⇒ y2 = 1/3

- Ratio of moles of P and Q in vapour phase = y1 : y2 = 2/3 : 1/3

⇒ P : Q = 2 : 1