Gas Mixture Mole Fractions

Mole fraction

A component's mole fraction is its amount divided by the total amount of all components in the mixture. The fractions sum to one. Enter between two and six component amounts. A missing component may have zero amount, but the total cannot be zero. Convert mass inputs to moles using their respective molar masses first. Mass percentages cannot be used directly as mole fractions.

Partial pressure

In a nonreacting ideal gas mixture, each partial pressure equals the component's mole fraction multiplied by the total absolute pressure. A mixture of 1 mol of A and 3 mol of B has fractions of 0.25 and 0.75. At a total pressure of 100 kPa, the partial pressures are 25 and 75 kPa. Condensation or reaction may change the amounts in the gas phase. Pressure system design and preparation of breathing gases are outside the scope of this calculator.

Check the mixture through its totals

Determine molar amounts for the same sample or flow duration. Adding one component's hourly amount to another's instantaneous amount does not define a common mixture. Omitting chemical names makes the calculation general, but does not establish ideal gas behaviour. Changing pressure while holding molar amounts fixed leaves fractions unchanged. Only partial pressures change scale.

Choose three component amounts of 4 mol, 1 mol and 3 mol at a total absolute pressure of 160 kPa. The total is 8 mol, giving fractions of 0.5, 0.125 and 0.375. Their sum is 1. Multiplying each fraction by total pressure gives partial pressures of 80 kPa, 20 kPa and 60 kPa. These sum to 160 kPa. The tool divides the amounts by their sum, so fractions always add to 1 and partial pressures add to the entered total pressure. These totals check hand arithmetic. They do not reveal a value placed in the wrong field or entered in the wrong unit.

Increasing every amount by the same factor leaves fractions unchanged. Changing one component also changes the other fractions because the denominator includes the whole mixture.

Frequently asked questions

How do you convert mole fraction to partial pressure?

For example, a gas mixture containing 2, 3 and 5 mol has a first component mole fraction of 0.2 and partial pressure of 40 kPa at 200 kPa absolute. Dalton’s relation multiplies mole fraction by total pressure, assuming an ideal nonreacting mixture without condensation or real gas corrections.

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