Pentane (C5H12) and hexane (C6H14) form an ideal solution. The vapor pressures of pentane and hexane at 25° C are 511 torr and 150 torr, respectively.

Question

CHAPTER 16The mole fraction of hexane in a pentane-hexane solution is 0.50. Calculate the mole fraction of pentane in the vapor that is in equilibrium at 25°C with this solution.

Answer 1

If the mole fraction of hexane in a hexane/pentane solution is 0.5, then the mole fraction of pentane must also be 0.5 (sum must = 1.0).

The mole fraction of hexane x vapor pressure of pure hexane = vapor pressure of hexane of the solution

The mole fraction of pentane x vapor pressure of pure pentane = vapor pressure of pentane of the solution

0.5 x 511 torr = 255.5 torr for hexane

0.5 x 150 torr = 75 torr for pentane

Total vapor pressure = 255.5 + 75 = 330.5

mole fraction pentane in the vapor = 75 torr / 330.5 torr = 0.23