Question

5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?

Options

• The solution shows positive deviation.

• The solution shows negative deviation.

• The solution is ideal.

• The solution has a volume greater than the sum of individual volumes.

Answer 1

The solution shows negative deviation.

Explanation:

X	Y
nx = 5	ny = 10
`P_x^\circ` = 63	`P_y^\circ` = 78

Total moles = n_x + n_y

= 5 + 10

= 15 

P_s(calculated) = `(n_x/(n_x + n_y)xx P_x^\circ) + (n_y/(n_x + n_y)xx P_y^\circ)`

= `(5/15 xx 63) + (10/15 xx 78)`

= `315/15 + 780/15`

= 21 + 52

= 73

P_s(observed) = 70

Hence, negative deviation.