When the non-volatile solute is added to the solvent the vapour pressure of the solution decreases. Now according to Raoult’s law, the pressure of the solvent (let P₁) in the solution is given by, 0 1 1 1 P x P SOLUTIONS 35 Where, 0 1P is the vapour pressure of pure solvent, 1 x is the mole fraction of a solute, and P₁ is the pressure of the solution. We can say that the vapour pressure of a non- volatile solute is directly proportional to the mole fraction of the solvent. It is, P x Thus, (a) is the correct option.
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