Question

A liquid mixture of two unknown liquids, A and B, has a total vapor pressure of 0.04 bar. The vapor pressure of pure A under the same conditions is 0.03 bar, and the vapor pressure of pure B under the same conditions is 0.05 bar. Using Raoult’s Law, what are the mole fractions of A and B in solution?

Answer 1

According to Raoult's law,

where p is the total pressure of the solution (mixture), , are, respectively, the vapor pressures of pure A and pure B under the same conditions, and , are the mole fractions of A and B, respectively.

Since the sum of the mole fractions of all components in a solution is equal to one (i.e ), we can write the above equation as:

Since we know values of total pressure and pressure of pure components, we can substitute these to get the value of as follows:

Now using

therefore, mole fraction of both A and B in the solution is 0.5