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Vapour Pressure
At a constant temperature, the pressure exerted by the vapours of a liquid on its surface when they (liquid and its vapours) are in equilibrium, is known as vapour pressure.
Factors affecting Vapour Pressure:
The process of evaporation depends on different factors:-
(a) Nature of the liquid
(b) Effect of temperature
1.0Vapour Pressure of Liquid Solution
Raoult's Law
According to this law, the partial pressure of any volatile constituents of a solution at a constant temperature is directly proportional to the mole fraction of that constituent in the solution.
Vapour Pressure of Liquid – Liquid Solution
Let PA and PB be the partial vapour pressures of two constituents A and B in solution and PA0 and PB0 the vapour pressures in pure state respectivity.
At constant temperature partial vapour pressure of the component is directly proportional to the mole fraction of the component in solution.
Let PA and PB be the partial pressures of two constituents A and B in solution and PA0 and PB0 the vapour pressures in pure state respectively.
Thus, according to Raoult's law [for volatile liquids]
...(1)
Partial pressure of A = mole fraction of
and
...(2)
Partial pressure of B = mole fraction of
If total pressure is PS, then
According to Dalton's law of partial pressure given below: (If A is more volatile than B (PA0<PB0 )
If total pressure be PS, then
Following conclusions can be drawn from equation
(i) Total vapour pressure over the solution can be related to the mole fraction of any one component.
(ii) Total vapour pressure over the solution varies linearly with the mole fraction of component A.
2.0Related Questionnaire
1 mole heptane (V.P. = 92 mm of Hg) is mixed with 4 mole Octane (V.P. = 31 mm of Hg), forming an ideal solution. Find out the vapour pressure of the solution.
Sol. Total mole = 1 + 4 = 5
Mole fraction of heptane =
Mole fraction of octane =
=43.2 mm of Hg.