A bottle is half-filled with a 50:50 (mole-to-mole) mixture of n-heptane (c7h16) and n-octane (c8h18) at 25°c. what is the mole ratio of heptane vapor to octane vapor in the air space above the liquid in the bottle? the vapor pressures of heptane and octane at 25°c are 31 torr and 11 torr, respectively.

Answer is: mole ratio of heptane vapor to octane vapor is 2,8 : 1.

p (C₇H₁₆) = 31 torr.

p (C₈H₁₈) = 11 torr.

x (C₇H₁₆) = 0,5.

x (C₈H₁₈) = 0,5.

p total = (x (C₇H₁₆) · p (C₇H₁₆)) + (x (C₈H₁₈) · p (C₈H₁₈)) = 0,5· 31 torr + 0,5·11 torr = 21 torr.

The mole fraction of heptane in the vapor is:

p (C₇H₁₆) : p total) = 15,5 torr : 21 torr = 0,74.

The mole fraction of octane in the vapor is:

p (C₈H₁₈) : p total = 5.5 torr : 21 torr = 0,26.

Mole ratio heptane vapor : octane vapor:

0,74 : 0,26 = 2,8.