Problem: At 40°C, heptane has a vapor pressure of about 91.5 torr and octane has a vapor pressure of about 31.2 torr. Assuming ideal behavior, what is the vapor pressure of a solution that contains twice as many moles of heptane as octane?A. 81.8 torrB. 51.3 torrC. 10.4 torrD. 71.4 torrE. 61.0 torr

FREE Expert Solution

We’re being asked to determine the vapor pressure of a solution contains twice as many moles of heptane as octane?


Recall that the vapor pressure of a solution can be given by Raoult’s Law.

The given solutions are composed of heptane and octane, which are both volatile. For a solution with volatile solute and solvent, Raoult’s Law is given as:


Psolution = (P°heptaneXheptane) + (P°octaneXoctane)


where P˚ = vapor pressure of pure component and χ = mole fraction of component. Recall that the mole fractions in a solution add up to 1. Since the solution is composed of heptane and octane, this means:


Xoctane = 1 - Xhexane


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Problem Details

At 40°C, heptane has a vapor pressure of about 91.5 torr and octane has a vapor pressure of about 31.2 torr. Assuming ideal behavior, what is the vapor pressure of a solution that contains twice as many moles of heptane as octane?

A. 81.8 torr

B. 51.3 torr

C. 10.4 torr

D. 71.4 torr

E. 61.0 torr

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