Question

1-propanol (P1° = 20.9 Torr at 25 °C) and 2-propanol (P2° = 45.2 Torr at 25 °C) form ideal solutions in all proportions. Let x1 and x2 represent the mole fractions of 1-propanol and 2-propanol in a liquid mixture, respectively, and y1 and y2 represent the mole fractions of each in the vapor phase. For a solution of these liquids with x1 = 0.210, calculate the composition of the vapor phase at 25 °C.

Answer 1

mole fraction of X1 = 0.21

mole fraction of X2 = 1-X1

                           = 1-0.21 = 0.79

1- propanol vapor pressure = mole fraction * pressure

                                      = 0.21*20.9

                                       = 4.389 torr

2- propanol vapor pressure   = 0.79*45.2

                                       = 35.708 torr

mole fraction of 1- propanol in vapor = 4.389/4.389+35.708

                                                   = 0.1094 torr

mole fraction of 2- propanol in vapor = 1-0.1094

                                                     = 0.8906 torr