Question

A mixture of ethyl alcohol and propyl alcohol has a vapour pressure of 290 mm at 300 K. The vapour pressure of propyl alcohol is 200 mm. If the mole fraction of ethyl alcohol is 0.6, its vapour pressure (in mm) at the same temperature will be ______.

Options

• 350

• 300

• 700

• 360

Answer 1

A mixture of ethyl alcohol and propyl alcohol has a vapour pressure of 290 mm at 300 K. The vapour pressure of propyl alcohol is 200 mm. If the mole fraction of ethyl alcohol is 0.6, its vapour pressure (in mm) at the same temperature will be 350.

Explanation:

Given: Total vapour pressure of mixture `(P_"total")` = 290 mm

Vapour pressure of propyl alcohol `(P_B^circ)` = 200 mm

Mole fraction of ethyl alcohol (x_A) = 0.6

Let `P_A^circ` be the vapour pressure of pure ethyl alcohol (to be found)

By using Raoult’s law

`P_"total" = P_A^circ * x_A + P_B^circ * x_B`

`200 = P_A^circ xx 0.6 + 200 xx 0.4`

`290 = 0.6 P_A^circ + 80`

`0.6 P_A^circ = 210`

`P_A^circ = 210/0.6`

`P_A^circ = 350` mm