Question

1 mole of liquid A and 2 moles of liquid B make a solution having a total vapour pressure of 40 torr. The vapour pressure of pure A and pure B are 45 torr and 30 torr, respectively. The above solution ______.

Options

• is an ideal solution

• shows positive deviation

• shows negative deviation

• is a maximum boiling azeotrope

Answer 1

1 mole of liquid A and 2 moles of liquid B make a solution having a total vapour pressure of 40 torr. The vapour pressure of pure A and pure B are 45 torr and 30 torr, respectively. The above solution shows positive deviation.

Explanation:

If the actual pressure is lower than the expected pressure, the solution will have a negative deviation. If the actual pressure is greater than the expected pressure, the solution will display a positive deviation. If the actual pressure equals the expected pressure, the solution will be ideal.

X_A = `1/(1 + 2) = 1/3`

X_B = `2/(1 + 2) = 2/3`

PT = `"P"_"A"^0 xx "A" + "P"_"B"^0 xx "B"`

= `45 xx 1/3 + 30 xx 2/3`

= 35 torr

However, the actual pressure i.e. 40 torr, is greater than the expected pressure. As a result, the solution will have a positive deviation.