Molarity vs. Molality: What's the Difference?

Molarity and molality differ by one letter, sound almost identical when spoken, and both describe how concentrated a solution is. It’s no wonder they get mixed up. But they measure different things, and choosing the wrong one gives wrong answers in exactly the problems where it matters.

The definitions side by side

	Molarity	Molality
Symbol	M (or c)	m (or b)
Definition	moles of solute ÷ litres of solution	moles of solute ÷ kilograms of solvent
Units	mol/L	mol/kg
Read as	“molar”	“molal”
Changes with temperature?	Yes, slightly	No

Two differences are packed into that table:

1. Molarity divides by a volume; molality divides by a mass.
2. Molarity uses the whole solution; molality uses only the solvent.

Why temperature matters

Most liquids expand when heated. Warm a litre of solution and it becomes, say, 1.005 litres, but the number of moles of solute hasn’t changed. So the molarity drops slightly. Cool it and the molarity rises.

Mass doesn’t change with temperature. A kilogram of water is a kilogram of water whether it’s at 5 °C or 80 °C. So molality is temperature-independent, which makes it the right choice whenever a calculation spans a range of temperatures.

Where each one is used

Molarity is the everyday lab unit. Volumetric flasks, pipettes and burettes all measure volume, so it’s natural to prepare and dispense solutions by molarity. Titrations, reaction stoichiometry in solution and pH calculations use molarity.

Molality is used for colligative properties — properties that depend on how many solute particles there are, not what they are:

• Boiling-point elevation: ΔT_b = K_b × m × i
• Freezing-point depression: ΔT_f = K_f × m × i

These calculations involve changing temperatures by definition, so a temperature-independent unit is essential. It’s also why salting icy roads and adding antifreeze to a car radiator are described in terms of molality.

Worked example: molality

20.0 g of glucose (C₆H₁₂O₆, 180.16 g/mol) is dissolved in 250 g of water. What is the molality?

• Moles of glucose: 20.0 ÷ 180.16 = 0.1110 mol
• Mass of solvent: 250 g = 0.250 kg
• Molality: 0.1110 ÷ 0.250 = 0.444 mol/kg

Notice we used the mass of the water only, not water plus glucose.

Worked example: freezing point

What is the freezing point of that solution? Water’s K_f is 1.86 °C·kg/mol, and glucose doesn’t split into ions, so i = 1.

ΔT_f = 1.86 × 0.444 × 1 = 0.826 °C

The solution freezes at about −0.83 °C.

For an ionic solute the van ’t Hoff factor i counts the particles: NaCl gives about 2 (Na⁺ and Cl⁻), CaCl₂ about 3. That’s why calcium chloride is so effective on icy roads.

Converting between them

You can’t convert molarity to molality with the concentration alone — you also need the solution’s density, to link volume to mass.

A 2.00 M NaCl solution has a density of 1.08 g/mL. What is its molality?

Take 1.000 L of solution:

• Mass of solution: 1000 mL × 1.08 g/mL = 1080 g
• Moles of NaCl: 2.00 mol → mass 2.00 × 58.44 = 116.9 g
• Mass of water: 1080 − 116.9 = 963.1 g = 0.9631 kg
• Molality: 2.00 ÷ 0.9631 = 2.08 mol/kg

When are they nearly equal?

For dilute aqueous solutions, one litre of solution is almost exactly one kilogram of water (water’s density is close to 1 g/mL, and a little solute barely changes that). So a 0.010 M solution is very close to 0.010 mol/kg. The two diverge as solutions get more concentrated, or when the solvent isn’t water.

Quick answers

Which is more accurate? Neither is more accurate; they measure different things. Molality is more stable, because it doesn’t change with temperature.

Why use solvent mass instead of solution mass? Because colligative formulas depend on the ratio of solute particles to solvent. It’s a definition, and it keeps those equations simple.

What’s an easy way to remember which is which? Molarity uses the volume of the whole solution, which you’d read off a flask marked in litres; molality uses the mass of the solvent, which you’d weigh on a balance. “Molarity — litres of solution; molality — kilograms of solvent.”

Related

For molarity calculations and dilutions, use the molarity calculator, and see molarity explained for the full method.