Question

How does molecular weight affect boiling point?

Answer 1

In the ABSENCE of other intermolecular force, the higher the molecular mass the greater the boiling point.

Explanation:

Look at the alkane series: methane, ethane, propane, butane, etc.

The longer the alkyl chain, the greater the degree of dispersion force between molecules, and thus the greater the boiling point. Of course, as a chemist, as a physical scientist, you should look up the boiling point of the straight chain alkanes. Of course, branched alkanes, have less degree of intermolecular interaction, so we have to be careful to compare apples with apples, and not with oranges.

Now compare the boiling point of water. This is certainly a low molecular mass compound; `18.0*g*mol^-1`, and yet water has high melting and boiling points. Here, of course, the potent intermolecular force of hydrogen bonding operates, which was certainly not the case for the alkane series.