Question

The vapor pressure of 1 M of KI solution is ________ 1 M of glucose solution. This is because vapor pressure depends only on the _________.

Which option correctly completes this statement?

the same as; concentration of the solute
less than; number of particles of solute
less than; molar mass of the solute
higher than; molar mass of the solute

C?

Answer 1

Here is a statement made to me 72 years ago by my freshman chemistry teacher. "When a non-volatile solute (KI and glucose are non-volatile solutes) is dissolved in a volatile solvent (water in this case), three things happen.
1. The freezing point is lowered.
2.The boiling point is raised.
3. The vapor pressure of the solvent is lowered.
The extent to which these three things happen depends SOLELY on the number of dissolved particles.

So 1 M solutions should have the same vapor pressure BUT KI dissociates into TWO particles while glucose does not.Since the KI solution has twice as many particles as the glucose solution then the KI soln will have a smaller vapor pressure.

Answer 2

Great post DrBob!