Mole Fraction Explained

Proportions of mixed reactants affect the outcome of any chemical reaction. One way of gauging the relative proportions of compounds with each other is to calculate the mole fraction.

Chemists have many units for quantifying the amount of various substances used in a chemical reaction. One of the most commonly used ones is the 'Mole'. A mole of a substance is the molecular weight of that compound expressed in grams, which is a unit of mass.

What's so special about specifying the quantity of a substance in terms of a mole? One fact, which makes it a convenient and special unit of use is that a mole contains exactly 6.023 x 10²³ molecules in it. So, when you say you have one mole of any compound, you know the exact number of molecules contained in it. It is a specification of mass, as well as quantity.

A solution generally consists of more than one chemicals mixed together. The mole fraction of a compound is the ratio of the number of moles of that compound, with the total number of moles of all the substances that are mixed together in that solution.

It is a pure number which provides you with exact information about what is the ratio of number of moles of that compound, with those of the moles of all the compounds put together. So, it specifies the 'Fraction' of moles of each compound, which mix together to form the entire mixture or solution.

Mole Fraction of Compound A = (Moles of Compound A)/(Total Number of Moles of All Compounds in the Mixture)

First part of the whole exercise is to note the masses of the compounds, which have been mixed together to form the solution. Then, you must calculate the molar masses of each compound from the knowledge of its molecular weight.

Next step would be to divide the masses of each of the mixed components by their respective molar masses, to get the number of moles of each. To get the mole fraction for any specific compound, divide the number of moles of that particular compound, by the total number of moles, which are a part of the solution.

Let me illustrate the calculation with an example. Suppose that you have 223.5 gm of KCl (Potassium Chloride) mixed in about 500 grams of water. Since 1,000 gm makes 1 liter of water, that is about ½ liter of water. What is the mole fraction of KCl in this solution?

The molar mass of water is 18 gm and that of KCl is 74.5 gm. From this information, calculation of number of moles of water and potassium chloride is possible.

Number of Moles of KCl = 223.5 gm/74.5 gm = 3 moles

Number of moles of Water = 500 gm/18 gm = 27.78 moles

Now that we know the number of moles of each of the components, the final calculation is simple:

Mole Fraction of KCl = (3) / (3 + 27.78) = 0.0975 (approx.)

This value provides you with an exact idea of the ratios in which reactants have been mixed together in any solution. It is one of the most useful concepts of stoichiometry.