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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
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Solution
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.
XA = `1/(1 + 2) = 1/3`
XB = `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.
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