# 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:

$\overline{){{\mathbf{P}}}_{{\mathbf{solution}}}{\mathbf{=}}{\mathbf{P}}{{\mathbf{°}}}_{{\mathbf{solvent}}}{{\mathbf{\chi }}}_{{\mathbf{solvent}}}}$

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:

χsolvent = 0.97 ###### 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?