Question

Ethanol and methanol form a nearly ideal solution.

(a) First of all, why is this expected?

(b) At 20 °C, the vapor pressure of ethanol is 5.93 kPa, and that of methanol is

11.83 kPa. Calculate the mole fractions of the two components if 100 g of

each are mixed.

(c) Calculate the partial pressures and the total vapor pressure of the solution.

(d) Calculate the mole fraction of methanol in the vapor.

Answer 1

(a) Ethanol and methanol form a nearly ideal solution.

This is because both have identical molecular structures. Hence, they have identical intermolecular forces.

In other words, the interactions between two ethanol molecules are same as the interactions between two methanol molecules. These are same as the interactions between one ethanol molecule and one methanol molecule. These interactions are hydrogen bonds.

(b) The molar masses of methanol and ethanol are 32 g/mol and 46 g/mol respectively.

100 g of methanol are mixed with 100 g of ethanol.

The number of moles of methanol

The number of moles of ethanol

The mole fraction of methanol

The mole fraction of ethanol

(c) The partial pressure of methanol is obtained by multiplying its mole fraction with vapour pressure of pure methanol.

The partial pressure of methanol

The partial pressure of ethanol is

The total vapour pressure of solution is equal to the sum of the vapour pressures of methanol and ethanol.

It is

(d) The mole fraction of methanol in vapour is the ratio of the partial pressure of methanol to the total vapour pressure. It is