question_answer 28)

 Heptane and octane form ideal solution. At 373 K, then vapour pressure of the two liquid components are 105.2 kPa and 46.8 kPa respectively. What will be the vapour pressure, in bar of a mixture of 25.0 g of heptane and 35.0 g of octane?

Answer:

Molecular mass of heptane (C7H16) = 7 x 12 + 16 x 1 = 100 Molecular mass of octane (C8H18) 8 x 12 + 18 x 1 = 114 Number of moles of heptane = Number of moles of octane =   Total number of moles = 0.25 + 0.307 = 0.557 Mole fraction of heptane, Mole fraction of octane, Now by Raoult's law, Partial vapour pressure of heptane in solution 105.2 x 0.449 = 47.23 kPa Partial vapour pressure of octane in solution Poctane = 46.8 x 0.551 = 25.79 kPa Hence, total vapour pressure of solution P,            = 73.02 kPa = 73.02 x 103 Pa = 0.73 bar ( 1 bar = 105 Pa)