Molality Calculator

The Molality Calculator finds molality (mol/kg) from moles of solute and solvent mass, and — optionally — the freezing point depression and boiling point elevation that molality causes.

What is molality?

Molality (m) is the number of moles of solute dissolved per kilogram of solvent (not per liter of solution, which is molarity):

m = n ÷ mass of solvent (kg)

where n is moles. If you only know a solute's mass, moles come from mass and molar mass:

n = mass ÷ molar mass

Because it's defined against solvent mass rather than solution volume, molality doesn't change with temperature the way molarity does — which is exactly why it's the concentration used in colligative-property formulas.

Freezing point depression and boiling point elevation

A dissolved solute lowers a solvent's freezing point and raises its boiling point, in proportion to molality:

ΔTf = i × Kf × m      (freezing point depression)
ΔTb = i × Kb × m      (boiling point elevation)

• Kf and Kb are the solvent's molal freezing-point-depression and boiling-point-elevation constants. For water, Kf = 1.86 °C·kg/mol and Kb = 0.512 °C·kg/mol.
• i is the van't Hoff factor — how many particles the solute splits into in solution. It's 1 for a non-electrolyte like sugar or urea, 2 for NaCl (Na⁺ + Cl⁻), 3 for CaCl2, and so on.

How to use it

1. Choose how you're giving the solute amount — directly in moles, or as a solute mass + molar mass (don't know the molar mass? Use the Molar Mass Calculator to derive it from a chemical formula).
2. Enter the solvent mass, in kg or g.
3. Leave "Also calculate freezing & boiling point shifts" checked to add ΔTf and ΔTb — pick Water for the standard constants, or Custom solvent to enter your own Kf/Kb, plus the van't Hoff factor for your solute.

Example

Dissolving 1 mol of NaCl (i = 2) in 2 kg of water:

m = 1 mol ÷ 2 kg = 0.5 mol/kg
ΔTf = 2 × 1.86 × 0.5 = 1.86 °C   → freezes at −1.86 °C instead of 0 °C
ΔTb = 2 × 0.512 × 0.5 = 0.512 °C → boils at 100.512 °C instead of 100 °C

Why use molality instead of molarity for colligative properties?

Molarity depends on solution volume, which expands and contracts with temperature — so a solution's molarity actually drifts as it heats up or cools down mid-experiment. Molality is defined against solvent mass, which doesn't change with temperature, making it the concentration that stays valid across the freezing/boiling range being calculated.

What if my solute doesn't fully dissociate?

Use a van't Hoff factor between 1 and the theoretical maximum — weak electrolytes only partially ionize, so their real i is lower than the ion count suggests. For a non-electrolyte (doesn't ionize at all, like glucose), leave it at 1.

To go the other direction — solve for molarity given a solute mass and solution volume instead — use the Molarity Calculator.

Privacy

All calculations run entirely in your browser. Your figures are never uploaded or stored.