The following argument is frequently used to explain the fact that the vapor pre

Question

The following argument is frequently used to explain the fact that the vapor pressure of the solvent is lower over a solution than over the pure solvent and that lowering is proportional to the concentration.

A dynamic equilibrium exists in both cases so that the rate at which molecules of solvent evaporate from the liquid is always equal to that at which they condense. The rate of condensation is proportional to the partial pressure of the vapor, whereas that of evaporation is unimpaired in the pure solvent but is impaired by the solute molecules in the surface of the solution. Hence the rate of escape is reduced in proportion to the concentration of the solute, and maintenance of equilibrium requires a corresponding lowering of the rate of condensation and therefore of the partial pressure of the vapor phase.

Explain why this argument is incorrect.

Explanation / Answer

For the vapor pressure eperiment. As the impurity which is a non-volatile component in a solution goes an sits in between the molecules of liquid. There are lesser number of molecules of pure liquid on the surface that can escape into the vapor phase. Thus at any given time, the pressure exerted by the molecules of vapor on the liquid would be lower than that of the pure solvent wherein, greater number of molecules exist in the vapor phase. The rate of condensation is dependent upon the solution and not on solute.

Thus the given explanation in incorrect and does not fit into the colligative property description of lowering of vapor pressure of solution over that of the solvent.

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