PPM to Mole Fraction Calculator

Mole fraction shows up in Raoult's law and vapor-pressure problems. For dilute aqueous solutions, it is approximately PPM ÷ (MW × 55,556) — this calculator runs it with your solute's molecular weight.

How to use this calculator

1. Enter PPM.
2. Enter solute molecular weight.
3. Get mole fraction.

The formula

In 1 kg water there are ~55.5 mol; dilute solute moles = PPM ÷ (MW × 1000); fraction = solute mol ÷ 55.5.

What the units mean

PPM (parts per million) — one part of solute per million parts of the total mixture. The workhorse unit for trace concentrations in water, air, and food.

Mole fraction — moles of one component divided by total moles. A pure number between 0 and 1, used in vapor-pressure and gas-solubility work.

Assumption: This conversion assumes a dilute water-like solution (density ≈ 1 kg/L). For concentrated solutions or other solvents, measure density and convert properly.

Worked example

Quick reference

Dilute aqueous approximation.

Where you will actually use this

Raoult's law, Henry's law, and colligative work all want mole fractions.

• Vapor pressure calculations
• Gas solubility
• Physical chemistry problems

Mistakes that give wrong answers

• Using it for concentrated solutions
• Wrong solvent molar mass

Key takeaways

• Formula: Mole fraction ≈ PPM ÷ (MW × 1,000,000 ÷ 18) [dilute water]
• Remember: Dilute aqueous approximation.
• Watch out for: Using it for concentrated solutions
• Next step: run your own numbers in the calculator above, then verify with a measurement — calculated values are only as good as the inputs.

Frequently asked questions

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