Question

At 20 C the vapor pressure of substance M is 520 torr; of substance N, 634 torr. Which substance will have the lower boiling point? The lower molar heat of vaporization?

Answer 1

Vapor pressure is one of the ways of indirectly ranking different molecules according to the strength of their intermolecular forces (IMFs). Molecules with strong IMFs hold onto each other well in the liquid phase; molecules with weak IMFs do not. When we know the vapor pressure of 2 different substances, the one with the stronger IMFs as a liquid will send fewer molecules into the gas phase as it evaporates (which creates the vapor pressure).

A molecule's boiling point is also related to the strength of the IMFs. The weaker IMFs will allow molecules of liquid to become gaseous more easily than a liquid with strong IMFs. Since M has a VP of 520, and N has a VP of 634, M has a lower VP, and it must have stronger IMFs, and will have a higher BP than N.

The same logic works for the molar heat of vaporization. It will take less energy to vaporize 1mol of N than it will for 1mol of M because of the stronger IMFs holding liquid M together.