Question

A solution contains naphthalene (C10H8) dissolved in hexane (C6H14) at a concentration of 11.92 % naphthalene by mass. Calculate the vapor pressure at 25 ∘C of hexane above the solution. The vapor pressure of pure hexane at 25 ∘C is 151 torr.

Answer 1

11.92 % naphthalene by mass. means 11.92 g naphthalene present in 100 g solution.

no of mole of naphthalene = w/mwt = 11.92/ 128.2 = 0.093 mol

no of mole of hexane = w/mwt = (100-11.92)/86.18 = 1.022 mol

molefraction of hexane = nhexane/ntotal

                                      = 1.022/(1.022+0.093)

                                       = 0.916

vapor pressure of solution(p) = Xhexane*P0hexane

            P0hexane = vapor pressure of pure hexane = 151 torr

              p = 0.916*151 = 138.316 torr