Molarity to Molality Converter
Convert solution molarity (mol/L of solution) to molality (mol/kg of solvent) from the solution density and the molar mass of the solute.
Last updated: October 2026
Molality
1.02 mol/kg
Formula below · 1 source (goldbook.iupac.org) · Updated Oct 2026
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About this calculator
Molarity (M) is moles of solute per liter of solution; molality (m) is moles of solute per kilogram of solvent. To convert, find how much solvent is in one liter of solution: its total mass is 1,000 × density (in g), and the solute accounts for M × molar mass of that. So:
m = 1,000 × M ÷ (1,000 × ρ − M × Mₛ)
where ρ is the solution density in g/mL and Mₛ the solute's molar mass in g/mol. Going the other way, M = 1,000 × ρ × m ÷ (1,000 + m × Mₛ). Use the density of the solution, not of pure water; tables list it by concentration and temperature.
Molality does not change with temperature because it is based on masses, which is why it is used for boiling-point elevation and freezing-point depression. For dilute water solutions the two values are close; they diverge as the solute takes up more of the solution's mass.
How to use
Enter the molarity, the solution density and the solute's molar mass. The default is 1 M sodium chloride with a density of 1.04 g/mL: 1,000 × 1 ÷ (1,040 − 58.44) = 1.02 mol/kg. A 2 M NaCl solution with density 1.08 g/mL is 2,000 ÷ (1,080 − 116.88) = 2.08 mol/kg. To check the reverse, 1.02 mol/kg at 1.04 g/mL gives 1,000 × 1.04 × 1.02 ÷ (1,000 + 1.02 × 58.44) = 1.0 M.
Frequently asked questions
Why do I need the solution density?
Molarity is per liter of solution and molality per kilogram of solvent. The density turns the liter into a mass, and subtracting the solute mass leaves the solvent mass. Without it the conversion cannot be made.
When are molarity and molality nearly equal?
In dilute water solutions near room temperature, where a liter of solution contains almost exactly a kilogram of water. At 0.01 M sodium chloride the difference is well under 1%.
How do I convert molality back to molarity?
Use M = 1,000 × ρ × m ÷ (1,000 + m × Mₛ). For 1.02 mol/kg NaCl at 1.04 g/mL: 1,000 × 1.04 × 1.02 ÷ 1,059.6 = 1.0 M.
Which should I use for freezing-point depression?
Molality. ΔT = i × K × m uses moles per kilogram of solvent because it does not shift with temperature or with the volume change on mixing.
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