What's the difference between molality and molarity?

Molality is moles of solute per kilogram of solvent; molarity is moles per litre of solution. Molality uses mass, which doesn't change with temperature, so it stays constant when a solution is heated or cooled.

When should I use molality?

For colligative properties — boiling-point elevation, freezing-point depression, osmotic pressure — and any work where temperature varies. Those equations are written in molality precisely because it's temperature-stable.

Is solvent mass the same as solution mass?

No. Solvent mass is just the dissolving liquid. Solution mass is solvent plus solute. For a 58.44 g solute in 1,000 g water, the solvent mass is 1,000 g and the solution mass is 1,058.44 g.

How close are molality and molarity for dilute water solutions?

Very close. For dilute aqueous solutions near room temperature, 1 L of solution is about 1 kg of water, so the two numbers nearly match. They diverge as concentration rises or the solvent isn't water.

How do I use molality to find a freezing point?

ΔT = i × Kf × molality, where Kf is the solvent's cryoscopic constant (1.86 °C·kg/mol for water) and i is the van 't Hoff factor (2 for NaCl). A 1 molal NaCl solution freezes about 3.7 °C lower than pure water.

Do I need the solution's density for molality?

No — that's the advantage. Molality needs only masses. Density is required to convert between molality and molarity, not to compute molality itself.

Molality Calculator

Why molality exists alongside molarity

Molarity is moles per litre of solution — convenient, because you can dispense a volume with a pipette. But a litre isn’t a fixed amount of stuff. Warm a solution and it expands; its molarity drops even though nothing was added or removed. Molality sidesteps that by measuring against the mass of solvent instead. A kilogram of water is a kilogram of water at any temperature, so molality never drifts.

That stability is why the equations for colligative properties — how much a dissolved substance raises a boiling point or lowers a freezing point — are all written in molality. It’s also why antifreeze and road-salt calculations use it.

The trap is the denominator. Molality is per kilogram of solvent — the dissolving liquid alone — not per kilogram of the whole solution.

The molality formula

is molality in mol/kg, is moles of solute, and are the solute’s mass and molar mass, and is the solvent mass in kilograms.

Worked example

The calculator’s defaults are 58.44 g of NaCl (molar mass 58.44 g/mol) in 1,000 g of water.

Moles of solute:

Solvent mass in kilograms: 1,000 ÷ 1,000 = 1.000 kg. Molality:

A 1 molal NaCl solution.

Molality Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

Molality in colligative-property work

For freezing-point depression, . With water’s °C·kg/mol and NaCl’s van ‘t Hoff factor :

Molality (NaCl)	Freezing point depression	Approx. freezing point
0.5 mol/kg	1.86 °C	−1.86 °C
1.0 mol/kg	3.72 °C	−3.72 °C
2.0 mol/kg	7.44 °C	−7.44 °C
3.0 mol/kg	11.16 °C	−11.16 °C

Where molality is used

Freezing-point depression and boiling-point elevation calculations.
Osmotic pressure and other colligative properties.
Cryoscopy to determine an unknown’s molar mass from how much it depresses a freezing point.
Any solution work across a temperature range, where a volume-based unit would drift.

Common mistakes

Using solution mass instead of solvent mass in the denominator.
Forgetting the van ‘t Hoff factor for salts that dissociate — NaCl gives 2 particles, CaCl₂ gives 3.
Treating molality and molarity as equal at high concentration or in non-aqueous solvents.

For the volume-based unit, see the molarity calculator. For the mixture-ratio unit used in gas and vapour work, the mole fraction calculator.

Molality from a percent-by-mass solution

Labels often give a mass percent. Assume 100 g of solution, then split it into solute and solvent. A 10% NaCl solution has 10 g of salt and 90 g of water:

The solvent mass is 90 g, not 100 g. Using the full 100 g would give 1.71 mol/kg, about 10% too low.

Converting molarity to molality

You need the solution’s density. For a 1.00 M NaCl solution with a density of about 1.04 g/mL, take one litre:

Solution mass: g.
Solute mass: g.
Solvent mass: g, or 0.9816 kg.

For dilute water solutions the two units are close. They drift apart as the concentration rises or the solvent’s density moves away from 1.

Boiling-point elevation

The boiling-point version of the colligative formula uses water’s °C·kg/mol. For the 1 mol/kg NaCl solution from the worked example, with :

So the solution boils at about 101.0 °C at sea level. That’s why salting pasta water barely changes its boiling point: a typical pinch gives a molality far below 1.

About the formula: Molality = Moles of solute ÷ Mass of solvent (kg). Solvent mass is entered in grams and converted internally.