Find the vapor pressure of a solution in which the mole fraction of HgCl2 is .163 at 25 degrees Celsius.

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For looking at vapor pressure of a solution, our goal is to compare the vapor pressure of the pure solvent to that of the solution.  Therefore, we need to know the mole fraction of the solvent and the vapor pressure of the pure solvent.  Since a solvent isn't mentioned, we can assume we are in an aqueous solution. 

At 25 degC, the vapor pressure of water is 1 atm. 

Psoln = xH2O * Psolvent

where Psoln is the vapor pressure of the solution, xH2O is the mole fraction of the solvent, and Psolvent is the vapor pressure of the pure solvent.

Since we know the mole fraction of the solute is 0.163.  Since mole fractions must sum to 1, the mole fraction of H2O is 1-0.163 = 0.837.

Psoln = (0.837)(1 atm)

Psoln = 0.837 atm

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