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:

$\overline{){{\mathbf{P}}}_{\mathbf{solution}\mathbf{}}{\mathbf{=}}{\mathbf{}}\mathbf{\left(}\mathbf{P}{\mathbf{\xb0}}_{\mathbf{heptane}}{\mathbf{X}}_{\mathbf{heptane}}\mathbf{\right)}{\mathbf{}}{\mathbf{+}}{\mathbf{}}\mathbf{\left(}\mathbf{P}{\mathbf{\xb0}}_{\mathbf{octane}}{\mathbf{X}}_{\mathbf{octane}}\mathbf{\right)}}$

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:

$\overline{){{\mathbf{X}}}_{\mathbf{octane}\mathbf{}}{\mathbf{=}}{\mathbf{}}{\mathbf{1}}{\mathbf{}}{\mathbf{-}}{\mathbf{}}{{\mathbf{X}}}_{{\mathbf{hexane}}}}$

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

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 professor is this problem relevant for?

Based on our data, we think this problem is relevant for Professor Schurmeier's class at UCSD.