Molality Calculator (from grams)
Work out molality (mol solute per kg solvent) from the mass of solute, its molar mass and the mass of solvent.
How it works
moles = mass ÷ molar mass · molality (m) = moles of solute ÷ kg of solvent
Molality measures concentration as moles of solute per kilogram of solvent — not per litre of solution, so unlike molarity it does not change with temperature. First convert the solute mass to moles using its molar mass, then divide by the solvent mass in kilograms. It is the concentration term used in boiling-point elevation and freezing-point depression.
Worked example
Dissolving 5.85 g of NaCl (molar mass 58.44 g/mol) in 1 kg of water gives 5.85 ÷ 58.44 = 0.100 mol, so the molality is 0.100 ÷ 1 = 0.100 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 uses mass, so it is unaffected by temperature-driven volume changes, which is why colligative-property formulas use it.
Do I use the mass of solvent or of solution?
Solvent only. If you know the mass of the whole solution, subtract the solute mass first to get the solvent mass before dividing.
What are the units?
Molality is expressed in mol/kg, often written with a lowercase m (e.g. 0.100 m). It is a different symbol from capital M used for molarity.
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