Donaldson R F Molarity & Molality Calculator
Two ways to measure concentration: per litre of solution, or per kilogram of solvent.
How this is worked out
Molarity (symbol M) is moles of solute per litre of solution (solute plus solvent together), while molality (symbol m) is moles of solute per kilogram of solvent only — a small but important difference. Molarity changes slightly with temperature, because the solution's volume expands or contracts; molality doesn't, because it's based on mass, which temperature doesn't change, so molality is the preferred unit for anything temperature-sensitive, like boiling-point elevation or freezing-point depression calculations.
Frequently asked questions
How do I find the moles of solute if I only have a mass?
Divide the mass (in grams) by the substance's molar mass (in g/mol) — this site's Molar Mass Calculator works out the molar mass of any chemical formula for you.
Are molarity and molality ever the same number?
Only approximately, and only for a dilute aqueous solution, where 1 litre of solution weighs close to 1 kilogram (water's density) — for anything concentrated, or any solvent other than water, the two values can differ noticeably.
Related tools: Molar Mass Calculator, pH & pOH Calculator, Chemical Equation Balancer.
Source / Last verified: standard general chemistry definitions (molarity = mol solute / L solution; molality = mol solute / kg solvent) — read/checked 1 October 2026.