Question

At a certain temperature the vapor pressure of pure heptane

C7H16

is measured to be

454.mmHg

. Suppose a solution is prepared by mixing

102.g

of heptane and

135.g

of chloroform

CHCl3

.

Calculate the partial pressure of heptane vapor above this solution. Be sure your answer has the correct number of significant digits.

Note for advanced students: you may assume the solution is ideal.

Answer 1

Number of moles of heptane = Amount of heptane added / Molar mass of heptane

Number of moles of heptane = 1.0178

Number of moles of chloroform = Amount of chloroform added / Molar mass of chloroform

Number of moles of chloroform = 1.1308

Total number of moles = 2.1486

Mole fraction of heptane = number of moles of heptane / total number of moles = 0.4737

According to Raoults law,

Partial pressure of heptane vapour above the solution = Mole fraction of heptane Vapour pressure of heptane in the pure state

                                                                                      = 454 0.4737 = 215.059 215 (After rounding off)

Partial pressure of heptane vapour above the solution is 215 mm Hg.