ChemBench

Molality Calculator

Moles of solute per kilogram of solvent.

Mass of solute (g)

Molar mass (g/mol)

Solvent mass (kg)

Molality

1.001 mol/kg

What you entered

Mass of solute (g): 58.5Molar mass (g/mol): 58.44Solvent mass (kg): 1

1
Moles = mass ÷ molar mass
58.5 ÷ 58.44= 1.001 mol
2
Molality = moles ÷ kg of solvent
1.001 ÷ 1= 1.001 mol/kg

Result

Molality: 1.001 mol/kg

This solution has a molality of 1.001 mol/kg (m).

Molality from mass, molar mass, and solvent mass

Molality is moles of solute per kilogram of solvent. Unlike molarity it uses solvent mass, so it does not change with temperature.

Solute mass	Molar mass	Solvent mass	Moles	Molality
58.44 g NaCl	58.44 g/mol	1 kg	1.0000	1.0000 m
58.44 g NaCl	58.44 g/mol	0.5 kg	1.0000	2.0000 m
10 g NaCl	58.44 g/mol	0.5 kg	0.1711	0.3422 m
180.16 g glucose	180.16 g/mol	1 kg	1.0000	1.0000 m
100 g glucose	180.16 g/mol	2 kg	0.5551	0.2775 m

Molality is the concentration used for boiling point elevation and freezing point depression, precisely because it is temperature-independent while molarity is not.

Why molality uses solvent mass, not solution volume

Unlike molarity, molality is based on the mass of solvent rather than the total solution's volume — this makes it independent of temperature, since mass doesn't change as liquids expand or contract with heat.

Where molality is required

Boiling point elevation and freezing point depression calculations specifically require molality rather than molarity, precisely because those effects need a temperature-independent concentration measure.

Frequently asked questions

I dissolved 20 g of sugar (molar mass 342.3 g/mol) in 250 g of water — what is the molality?

Moles = 20/342.3 = 0.0584 mol. Solvent mass = 0.250 kg. Molality = 0.0584/0.250 = 0.234 m.

Why use molality instead of molarity?

Molality uses solvent mass (which does not change with temperature) instead of solution volume (which does). The boiling point elevation and freezing point depression calculators both require molality for this reason.

Are molality and molarity ever the same number?

They are nearly identical for very dilute aqueous solutions at room temperature, because 1 L of dilute aqueous solution weighs close to 1 kg. The values diverge significantly for concentrated solutions or non-aqueous solvents.