Case 1
- Moles of component A (mol)
- 2mol
- Moles of component B (mol)
- 3mol
0.4000
Open with these valuesMole Fraction Calculator
0.4000
Result: 0.4000The mole fraction of A is its amount divided by the total amount: 2 mol of A with 3 mol of B gives 2 ÷ 5 = 0.4. It is dimensionless, always between 0 and 1, and the other component takes the rest — x_B is simply 1 − x_A, here 0.6.
Worked examples
Case 2
- Moles of component A (mol)
- 1mol
- Moles of component B (mol)
- 1mol
0.5000
Open with these valuesCase 3
- Moles of component A (mol)
- 3mol
- Moles of component B (mol)
- 1mol
0.7500
Open with these valuesHow it's calculated
x_A = nA ÷ (nA + nB)
- StepEnter the amount of the component you are asking about, in moles.
- StepEnter the amount of the other component, also in moles.
- ResultRead x_A; the second component holds the rest, 1 − x_A.
What this number means
Dimensionless, always between 0 and 1
Moles divided by moles cancel, so x_A carries no unit. 2 mol of A with 3 mol of B gives 2 ÷ 5 = 0.4.
The fractions add up to 1
In a two-component mixture x_B = 1 − x_A, so 0.4 for A means 0.6 for B. The calculator shows x_A; the other component holds the rest.
Moles, not grams
The formula counts amount of substance, never mass. If you only have masses, convert each one to moles with its molar mass first.
To mole percent, multiply by 100
A mole fraction of 0.4 is 40 mol%. It is the same composition, written out of a hundred instead of out of one.
Commonly misread
I have 2 g of A and 3 g of B, so x_A is 0.4.
That would be a mass fraction. Convert both masses to moles with their molar masses first, then divide.
The mole fraction carries the unit mol.
It carries none, because moles divided by moles cancel. The result is a bare number between 0 and 1.
With 3 mol of A and 1 mol of B, x_A is 3.
The divisor is the total, not the other component: 3 ÷ 4 = 0.75. x_B is then 0.25.
Reference table
| Moles A, moles B | The mixture | Mole fraction of A |
|---|---|---|
| 0, 5 | no A at all | 0.0000 |
| 0.145, 0.855 | a trace component | 0.1450 |
| 0.5, 1.5 | one part A to three parts B | 0.2500 |
| 2, 3 | two moles of A, three of B | 0.4000 |
| 1, 1 | equal amounts of both | 0.5000 |
| 3, 1 | three parts A to one part B | 0.7500 |
| 10, 0 | pure A | 1.0000 |
Questions
How do I calculate a mole fraction?
Divide the moles of one component by the total moles in the mixture: x_A = nA ÷ (nA + nB). 2 mol of A with 3 mol of B makes 5 mol in total, so x_A is 0.4.
Do the mole fractions add up to 1?
Yes, always. In a two-component mixture x_B = 1 − x_A, so a mole fraction of 0.4 for A means 0.6 for B.
How do I turn a mole fraction into a percentage?
Multiply by 100. A mole fraction of 0.4 is 40 mol%, which is the same composition written out of a hundred instead of out of one.
Which unit does a mole fraction have?
None — it divides moles by moles, so the units cancel. If you only have masses, convert each one to moles with its molar mass first.
What is a mole fraction used for?
It is the natural way to state the composition of gas mixtures and ideal solutions. It carries Dalton's law of partial pressures, where the partial pressure is the mole fraction times the total pressure, and Raoult's law for vapour pressure.
Sources and last check
Information, not professional advice.