A liquid mixture consisting of 70 mole% water and 30 mole% ethanol is placed in a container where the pressure is slowly decreased while the temperature is held constant at 20C until the first amount of vapor forms.

a) Determine the pressure (mm Hg) when the mixture will first vaporize.

b) Calculate the mass composition of the initial amount of vapor.

The answers are bellow, I just don't know how to get there.

Problem 1a: 25.4 mm Hg Problem 1b: 0.265 g water/g