Question

Cyclohexane (C6H12) has a vapor pressure of 99.0 torr at 25°C. What is the vapor pressure depression (in torr) of cyclohexane above a solution of 10.0 g naphthalene (C10H8) in 70.0 g cyclohexane at 25°C?

Answer 1

moles of solute =n1 = 10 / 128 = 0.0781

moles of solvent = n2 = 70 / 84 = 0.833

mole fraction of solute = n1 / n1 + n2

                                    = 0.0781 / 0./0781 + 0.833

                                   = 0.0857

vapor pressure depression = mole fraction of solute x pure pressure

                                            = 0.0857 x 99

                                           = 8.48 tott