Problem: At 35°C the vapor pressure of pure ethanol (C2H5OH, M = 46.1 g/mol) is pE° = 100.0 torr, and the vapor pressure of pure 1-propanol (C3H7OH, M = 60.1 g/mol) is pP° = 37.6 torr.  A solution at equilibrium at this temperature has a total pressure p = 84.2 torr.  Assuming the solution is ideal, what is the partial pressure of ethanol above the solution?

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We’re being asked to determine the partial pressure of ethanol (C2H5OH) above a solution. Recall that the vapor pressure of a solution can be given by Raoult’s Law.


The given solutions are composed of ethanol and 1-propanol, which are both volatile. For a solution with volatile solute and solvent, Raoult’s Law is given as:



where P˚ = vapor pressure of pure component and χ = mole fraction of component. Recall that the mole fractions in a solution add up to 1. Since the solution is composed of ethanol and 1-propanol, this means:



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Problem Details

At 35°C the vapor pressure of pure ethanol (C2H5OH, M = 46.1 g/mol) is pE° = 100.0 torr, and the vapor pressure of pure 1-propanol (C3H7OH, M = 60.1 g/mol) is pP° = 37.6 torr.  A solution at equilibrium at this temperature has a total pressure p = 84.2 torr.  Assuming the solution is ideal, what is the partial pressure of ethanol above the solution?

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