Question

A solution is composed of 1.90 mol cyclohexane (P∘cy=97.6 torr) and 2.20 mol acetone (P∘ac=229.5 torr). What is the total vapor pressure Ptotal above this solution?

Answer 1

According to Raoult’s law, when you have mixture of liquid the total vapor pressure is calculated by the following equation.

Total vapor pressure = vapor pressure of component 1 + vapor pressure of component 2

Vapor pressure of a component = mole fraction * partial vapor pressure

Moles of cyclohexane = 1.90 mol

Moles of acetone = 2.20 mol

Total number of moles = 1.90 mol + 2.20 mol = 4.10 mol

Mole fraction of cyclohexane = moles of cyclohexane/total number of moles

                                                 = 1.90 mol/4.10 mol

                                                 = 0.46

Mole fraction of acetone = moles of acetone/total number of moles

                                          = 2.20 mol/4.10 mol

                                          = 0.54

Vapor pressure of cyclohexane = 0.46 * 97.6 torr

                                                   = 44.89 torr

Vapor pressure of acetone = 0.54 * 229.5 torr

                                           = 123.93 torr

Total vapor pressure = 44.89 torr + 123.93 torr = 168.82 torr

Total vapor pressure of the solution is 168.82 torr.