1. A solution contains 50.0 g of heptane (C 7 H 16 ) and 50.0 g of octane (C 8 H

Question

1.       A solution contains 50.0 g of heptane (C7H16) and 50.0 g of octane (C8H18) at 250C. The vapor pressures of pure hexane and pure octane at 250C are 45.8 torr and 10.9 torr, respectively.

Assuming ideal behavior , answer the following:

a.       What is the vapor pressure of each of the solution components in the mixture?

b.      What is the total pressure above the solution?

c.       What is the composition of the vapor in mass percent?

d.      Why is the composition of the vapor different from the composition of the solution?

Explanation / Answer

n heptane = 50 / 100 = 0.5 n octane = 50 / 114 = 0.439

a) X heptane = 0.5 / 0.939 = 0.532 X octane = 0.468

So P heptane = 0.532 * 45.8 = 24.3656 torr P octane = 0.468 * 10.9 = 5.1012 torr

b) total pressure = 23.3656 + 5.1012 = 28.4668 torr

c) X heptane in vapour = 23.3656 / 28.4668 = 0.821 X octane in vapour = 0.179

d) composition of vapour is different from solution because pure vapour pressure of hexane and octane are different .

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