Mol Fractions

The mol fraction is a very powerful tool when trying to understand what's going on in a mixture of compounds. When you're comparing the effect of the components of a mixture, the most useful way is to consider the proportions of molecules in the mix, which is done by measuring in mols, rather than conventional weights or volumes.

The mol fraction of a component of a mixture is the ratio of the number of mols of that component to the total number of mols of all the substances. As an example, the mol fraction of Me mols of pure ethanol mixed with Mw mols of pure water is Me/(Me+Mw). It doesn't matter what the molecular weight of those compounds are, nor their density or any other measure - the only important factor is the numbers of molecules. We'll be using the mol fraction quite a bit from now on, so let's define some terms now.

In liquids, the mol fraction of a compound will be referred to by a capital X with a subscript to indicate which compound we're talking about. For example, Xe for liquid ethanol and Xw for liquid water. So Xe = Me/(Me+Mw) and Xw = Mw/(Me+Mw).

In gases and vapors, the mol fraction of a compound will be referred to by a capital Y with a subscript to indicate which compound we're talking about. For example, Ye for ethanol vapor. So Ye = Me/(Me+Mw) or Yw = Mw/(Me+Mw).

This may seem a bit confusing at first, but it helps you see at a glance whether we are discussing a liquid or a vapor.

Try this sum yourself: in any two compound mix, Xa +Xb =1 and Ya + Yb = 1 This special relationship will be very useful in the following discussions.