We are asked to **calculate the mass % of octane (C _{8}H_{18})** in the vapor of the solution with heptane given the mass and pressure of each gass.

Recall that the ** vapor pressure of a gas (P_{Gas})** is given by:

$\overline{){{\mathbf{P}}}_{{\mathbf{gas}}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}{{\mathbf{\chi}}}_{{\mathbf{gas}}}{\mathbf{P}}{{\mathbf{\xb0}}}_{{\mathbf{gas}}}}$

where **χ**** _{Gas}** = mole fraction of the gas and

The ** mole fraction of the gas** is then given by:

$\overline{){{\mathbf{\chi}}}_{{\mathbf{Gas}}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}\frac{\mathbf{moles}\mathbf{}\mathbf{gas}}{\mathbf{total}\mathbf{}\mathbf{moles}}}$

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

A solution contains 40.0g of C_{7}H_{16} and 60.0g of C_{8}H_{18} at 25°C. The vapor pressure of C_{7}H_{16} and C_{8}H_{18} at 25°C are 45.8 torr and 10.9 torr respectively. What is the composition of the vapor in mass% of C_{8}H_{18}?

A. 10.3%

B. 26.3%

C. 60.0%

D. 55.1%

E. none of the above

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the The Colligative Properties concept. You can view video lessons to learn The Colligative Properties. Or if you need more The Colligative Properties practice, you can also practice The Colligative Properties practice problems.

What is the difficulty of this problem?

Our tutors rated the difficulty of*A solution contains 40.0g of C7H16 and 60.0g of C8H18 at 25°...*as high difficulty.