Problem: The normal boiling point of ethanol, CH3CH2OH, is 78.4 C. When 8.65 g of a soluble nonelectrolyte is dissolved in 100.0 g of ethanol at that temperature, the vapor pressure of the solution is 7.40 x 102 torr. What is the molar mass of the solute?

FREE Expert Solution

We’re being asked to determine the molar mass of the unknown solute in the ethanol solution


We’re given the vapor pressure of the solution, 7.40×102 torr, and the vapor pressure of pure ethanol, 760 torr. The vapor pressure of a solution can be given by Raoult’s Law:


Psolution=P°solventχsolvent


where P˚ = vapor pressure of solvent and χ = mole fraction of solvent


We shall follow these steps to determine the molar mass of the unknown solute:

Step 1: Calculate the mole fraction of ethanol

Step 2: Calculate the mole of unknown solute.

Step 3: Calculate the molar mass of the unknown solute.


Step1. The mole fraction of ethanol is:


Psolution=P°solventχsolvent7.40×102 torr760 torr =(760 torr )(χsolvent)760 torr 

χsolvent = 0.97 


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Problem Details

The normal boiling point of ethanol, CH3CH2OH, is 78.4 C. When 8.65 g of a soluble nonelectrolyte is dissolved in 100.0 g of ethanol at that temperature, the vapor pressure of the solution is 7.40 x 102 torr. What is the molar mass of the solute?

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