Percent by Mass to Molality Calculator
Convert a solution’s mass percent (w/w) into molality (mol solute per kg solvent) from the solute’s molar mass — no solution density needed.
How it works
molality m = (1000 × P) ÷ (M × (100 − P)) · where P = mass %, M = molar mass (g/mol)
Molality counts moles of solute per kilogram of solvent, not per litre of solution, which makes it independent of temperature. Starting from a convenient 100 g of solution, the solute mass is P grams and the solvent mass is (100 − P) grams. Divide the solute mass by its molar mass to get moles, then divide by the solvent mass in kilograms. Because it uses solvent mass rather than solution volume, you never need the solution’s density.
Worked example
For a 10% (w/w) NaCl solution (molar mass 58.44 g/mol): in 100 g there are 10 g of NaCl = 10 ÷ 58.44 = 0.1711 mol, dissolved in 90 g = 0.090 kg of water. Molality = 0.1711 ÷ 0.090 = 1.90 mol/kg.
Sources: Molality — Wikipedia
Frequently asked questions
How is molality different from molarity?
Molality is moles of solute per kilogram of solvent; molarity is moles per litre of solution. Molality does not change with temperature because it uses mass, while molarity does because liquid volume expands when heated.
Why can’t the mass percent be 100%?
At 100% there is no solvent left, so molality would divide by zero and be undefined. The calculator caps the input just below 100%.
Do I need the solution’s density?
No. Converting mass percent to molality only uses the solute and solvent masses, both of which come straight from the percentage. Density is only needed when you want molarity, which is per unit volume.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.