Mole fraction of solids and pure liquids

[Monkeymaniac]

Why is the mole fraction of solids and pure liquids equal to 1? Isn't mole fraction = mole solute / mole of solute and solvent? I read that it is why we are excluding the concentratino of solids or pure liquids from the equation for calculating eq constants. Any ideas?

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[jaxasp]

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).

 

[G1SG2]

Why is the mole fraction of solids and pure liquids equal to 1? Isn't mole fraction = mole solute / mole of solute and solvent? I read that it is why we are excluding the concentratino of solids or pure liquids from the equation for calculating eq constants. Any ideas?

It's not moles of solute/moles of solute and solvent, its moles of what you WANT/moles of solute plus solvent. So if you want the mole fraction of solvent (and remember, solvent is the substance that you have the MOST of), it will be moles solvent/moles of solute + solvent. We assume that the amount of solvent is so great that the mole fraction would be one. Think of it this way:

solvent=LOTS, solute=little

mole fraction of solvent=moles of solvent/moles of solvent+ moles of solute

which gives us

LOTS/LOTS + Little =(approximately equal to) LOTS/LOTS=1. Kinda like ignoring x in the equilibrium expression:

x^2/3-x=Keq. Upon ignoring x, we get x^2/3=Keq.