Let's assume the partial pressure of the solution is 24.7 mmHg, in which case its vapor pressure is 21.24 mmHg.
This value indicates the pressure at which the solvent molecules in the solution will evaporate. The value is determined by simply multiplying the partial pressure by the mole fraction. So, multiplying 0.86 by 24.7, you get 21.24 mmHg.
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