Mole Fraction Formula and Calculator
Mole Fraction Formula and Calculator
Calculate the mole fractions of solute and solvent from their masses, and how they relate to vapour pressure (Raoult law).
What is mole fraction?
The mole fraction (X) is the ratio of the moles of one component to the total moles in a mixture. It has no unit, lies between 0 and 1, and the mole fractions of all components add up to 1. It does not change with temperature, so it is used for colligative properties and gas mixtures.
Mole fraction formula
Worked examples
Example 1: glucose in water
18 g of glucose (180 g/mol) is dissolved in 90 g of water (18 g/mol). Find each mole fraction.
- n glucose = 0.1 mol; n water = 5 mol
- X glucose = 0.1 ÷ 5.1 = 0.0196
- X water = 0.9804
Answer: 0.0196 and 0.9804.
Example 2: 20% urea by mass
What is the mole fraction of urea (60 g/mol) in a 20% w/w solution?
- In 100 g: 20 g urea and 80 g water
- n urea = 0.333 mol; n water = 4.444 mol
- X urea = 0.333 ÷ 4.777 = 0.0698
Answer: 0.0698.
Example 3: vapour pressure lowering
Pure water at 25 °C has a vapour pressure of 23.76 mmHg. What is it for the solution in example 1?
- P = X water × P° = 0.9804 × 23.76 = 23.29 mmHg
Answer: 23.29 mmHg (a drop of 0.47 mmHg).
Frequently asked questions
Does mole fraction have a unit?
No, it is a ratio of moles to moles. It is sometimes given as mole percent (X × 100%).
Why do mole fractions add up to 1?
Each is a share of the same total number of moles.
Where is mole fraction used?
In Raoult law, partial pressures (Dalton law: P gas = X gas × P total) and the thermodynamics of mixtures.
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