The mol fraction of solids can be generalized to being one, but not for soluble liquids. if you add 1 mol of ethanol to 1 mol of methanol, both mol fractions will be .5. I think you are specifically talking about equilibrium constants, in which case you are not looking at mol fractions, but rather activity. I don't think you need to understand activity for the mcat, but I'll try to give you a very simplified break down.

Aqueous activity = molarity

Gaseous activity = partial pressure

Liquid activity = mol fraction if soluble, 1 if not soluble (i.e. water and oil)

Solid = almost always 1 because the crystal lattice will not be substituted

You can only ignored H2O in equilibrium constants with small solute concentrations, think about a .1 M NaCl solution in water. Water's STP molarity it 55, so the new mol fraction is 55/55.1, very close to 1 (sorry too lazy to use windows calculator).