What Is a Mole Fraction?

Mole fraction is a unit of concentration, defined to be equal to the number of moles of a component divided by the total number of moles of a solution. Because it is a ratio, mole fraction is a unitless expression. The mole fraction of all components of a solution, when added together, will equal 1.

Mole Fraction Example

In a solution of 1 mol benzene, 2 mol carbon tetrachloride, and 7 mol acetone, the mole fraction of the acetone is 0.7. This is determined by adding up the number of moles of acetone in the solution and dividing the value by the total number of moles of components of the solution:

Number of Moles of Acetone: 7 moles

Total Number of Moles in Solution = 1 moles (benzene) + 2 moles (carbon tetrachloride) + 7 moles (acetone)
Total Number of Moles in Solution = 10 moles

Mole Fraction of Acetone = moles acetone / total moles solution
Mole Fraction of Acetone = 7/10
Mole Fraction of Acetone = 0.7

Similarly, the mole fraction of benzene would be 1/10 or 0.1 and the mole fraction of carbon tetrachloride would be 2/10 or 0.2.