Question

Which substance in each pair of liquids would you expect to be less volatile? Why? butane or water, NH3 or NH2OH, CH3OH or CH4

Answer 1

The less volatile liquids in each pair are water, NH₂OH, and CH₃OH.

In each case, the reason is an especially strong type of dipole-dipole force — hydrogen bonding.

Hydrogen bonds form when you have a negative O, N, or F atom in one molecule and a positive H atom attached to an O, N, or F atom in another molecule.

As the attractive forces between molecules increase, the molecules are less likely to escape into the gas phase. The compounds become less volatile.

Butane vs. water

Water has strong hydrogen bonds. Each water molecule can H-bond to four other water molecules.

Thus, water is much less volatile than butane.

NH₃ vs. NH₂OH

Both NH₃ and NH₂OH can form hydrogen bonds.

Each NH₃ molecule can H-bond to four other molecules.

Each NH₂OH molecule can bond to six other molecules.

Thus, NH₂OH is less volatile than NH₃.

CH₃OH vs. CH₄

CH₃OH has H-bonds. Thus, CH₃OH is less volatile than CH₄.

Hope this helps.