🧫 Molality Calculator

m = moles of solute / mass of solvent (kg)

Solve for

Moles of solute (mol)

Mass of solvent (kg)

Molality (mol/kg)

—mol/kg

Molality

How to Use This Calculator

This tool calculates molality (m), moles of solute, or mass of solvent when you provide the other two values. Select what you want to solve for, fill in the known quantities, and click Calculate. The mass of solvent must be in kilograms, so convert grams by dividing by 1000 before entering.

Choose what to solve for: molality, moles of solute, or mass of solvent.

Enter the moles of solute. If you have grams, divide by the molar mass first (e.g. 36 g glucose / 180 g/mol = 0.2 mol).

Enter the mass of solvent in kilograms. Use only the solvent mass, not the total solution mass.

Read off the molality in mol/kg, or the missing quantity you selected.

The Molality Formula

m = n / kg_solvent ΔTb = Kb × m (boiling point elevation) ΔTf = Kf × m (freezing point depression) Kf(water) = 1.86 °C·kg/mol Kb(water) = 0.512 °C·kg/mol

Here m is molality in mol/kg, n is moles of solute, and kg_solvent is the mass of solvent only (not the solution). The molality does not change with temperature because mass is independent of temperature, unlike volume.

Worked Examples

36 g glucose (180 g/mol) in 200 g water0.2 mol / 0.200 kg = 1.00 mol/kg

Boiling point elevation of 1.00 mol/kg aqueous solutionΔTb = 0.512 × 1.00 = 0.512 °C above 100 °C

Freezing point depression for same solutionΔTf = 1.86 × 1.00 = 1.86 °C below 0 °C

11.7 g NaCl (58.44 g/mol) dissolved in 500 g water0.200 mol / 0.500 kg = 0.400 mol/kg

Where This Calculation Comes Up

Molality is the preferred concentration unit any time temperature varies during an experiment. When you calculate boiling point elevation or freezing point depression in a physical chemistry lab, you need molality rather than molarity because the volume of a solution expands when heated and contracts when cooled. Using molarity in those formulas would give you a slightly wrong answer that changes with temperature. A 1.00 mol/kg NaCl solution boils about 1.02 °C higher than pure water (the van't Hoff factor for NaCl is approximately 1.9, so ΔTb = 0.512 × 1.00 × 1.9 = 0.97 °C).

You will also meet molality in cryoscopy, the technique for measuring molar mass by freezing point depression. Dissolve a known mass of an unknown compound in a weighed amount of a solvent like camphor (Kf = 37.7 °C·kg/mol) or cyclohexane (Kf = 20.0 °C·kg/mol), measure the freezing point drop, and back-calculate the molar mass. It is a classic general chemistry lab and shows up frequently on standardized exams as a multi-step calculation.

Frequently Asked Questions

What is molality?

Molality (m) = moles of solute / mass of solvent in kg. Unlike molarity, it doesn't change with temperature.

When is molality preferred over molarity?

For colligative property calculations (ΔTb = Kb·m, ΔTf = Kf·m) and when temperature changes are involved.

What is the molal boiling point elevation constant Kb?

For water, Kb = 0.512 °C·kg/mol. A 1 m aqueous solution boils 0.512°C higher than pure water.

How does molality differ from molarity?

Molarity uses volume of solution; molality uses mass of solvent only. Mass doesn't change with temperature.

Example: 36 g of glucose (180 g/mol) in 200 g water?

n = 36/180 = 0.2 mol; m = 0.2 mol / 0.200 kg = 1.0 mol/kg