Problem: A solution contains 40.0g of C7H16 and 60.0g of C8H18 at 25°C. The vapor pressure of C7H16 and C8H18 at 25°C are 45.8 torr and 10.9 torr respectively. What is the composition of the vapor in mass% of C8H18?A. 10.3% B. 26.3% C. 60.0% D. 55.1% E. none of the above

FREE Expert Solution

We are asked to calculate the mass % of octane (C8H18) in the vapor of the solution with heptane given the mass and pressure of each gass.


Recall that the vapor pressure of a gas (PGas) is given by:


Pgas = χgasP°gas


where χGas = mole fraction of the gas and gas = vapor pressure of the pure gas


The mole fraction of the gas is then given by:


χGas = moles gastotal moles



In this case, we will follow these steps:

Step 1. Find the moles of octane and heptane
Step 2. Find the mole fraction of octane and heptane
Step 3. Vapor pressure of octane and heptane
Step 4. Calculate for the mass percent of octane

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Problem Details

A solution contains 40.0g of C7H16 and 60.0g of C8H18 at 25°C. The vapor pressure of C7H16 and C8H18 at 25°C are 45.8 torr and 10.9 torr respectively. What is the composition of the vapor in mass% of C8H18?

A. 10.3% 

B. 26.3% 

C. 60.0% 

D. 55.1% 

E. none of the above

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