Chemistry · Concentration
Molality and mole fraction calculator.
Molality, mole fraction and mass percent from grams of solute and solvent. Add a solution density to get molarity too.
Inputs
Result
Enter your values.
Molality and mole fraction formulas
Molality (m) = moles of solute / kilograms of solvent. Because it uses mass, it does not change with temperature the way molarity does. Mole fraction (X) = moles of a component / total moles of all components, so the solute and solvent fractions add to 1. Mass percent = mass of solute / mass of solution x 100.
Worked example. Dissolve 58.44 g of NaCl (58.44 g/mol, so 1.000 mol) in 1000 g of water (1000 / 18.015 = 55.51 mol). Molality = 1.000 mol / 1.000 kg = 1.000 m. Mole fraction of NaCl = 1.000 / (1.000 + 55.51) = 0.01770. Mass percent = 58.44 / 1058.44 x 100 = 5.521%.
The mole fraction here counts formula units as entered. It does not split NaCl into Na+ and Cl- ions; for colligative-property work, multiply by the van't Hoff factor separately. To convert to molarity you need the density of the finished solution, since molarity needs the solution volume.
Frequently asked questions
What is the formula for molality?
Molality = moles of solute divided by kilograms of solvent (not solution).
What is the difference between molality and molarity?
Molarity is moles per litre of solution and changes with temperature. Molality is moles per kilogram of solvent and does not.
How do I calculate mole fraction?
Divide the moles of one component by the total moles of every component. Convert grams to moles with each molar mass first.
Can I convert molality to molarity?
Yes, if you know the density of the solution. Enter it above and the tool finds the solution volume and the molarity.
Sources
Checked
- OpenStax Chemistry 2e, 11.4 Colligative Properties (molality, mole fraction, van't Hoff factor)
- OpenStax Chemistry 2e, 3.3 Molarity
- CIAAW: Abridged Standard Atomic Weights 2024 (molar masses)
Runs entirely in your browser. Nothing you type is sent or stored.
Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.