A sample of cyclohexane  $(C_6{H}_{12},bp={81}^{\circ }C)$ is introduced into an evacuated container at ${40}^{\circ }C$ . One-third of the cyclohexane by mass is in the form of the liquid and two-thirds is in the form of the vapour. The volume of the container is then doubled, maintaining the temperature at  ${40}^{\circ }C$. What happens to the pressure as the volume is doubled?

Imagine a system with 1 mol of liquid and 2 mol of vapor, and a volume of V. Then we double the volume to 2V. If there was liquid remaining, the pressure would be constant (held at the vapor pressure) and the same as before, and there would be 4 mol of vapor, but there are only 3 mol of substance total. So, all liquid is actually vaporized, and the pressure is only $\frac{3}{4}$ of what it was before as the initial pressure is proportional to 2 mol/V and the final one to be 3 mol/2V, i.e., a  $25\%$ decrease.