Molarity vs Molality
By the Periodixy Editorial Team · Last reviewed July 10, 2026
Molarity (M) and molality (m) sound alike, use similar symbols, and both measure concentration — which makes them one of chemistry's most reliable sources of mixed-up homework answers. The difference is the denominator: molarity divides by litres of finished solution, molality by kilograms of solvent. That small change decides whether your concentration drifts with temperature.
The two definitions
Molarity (M)
M = moles of solute ÷ litres of solution
Molality (m)
m = moles of solute ÷ kilograms of solvent
Note both differences: molarity uses volume where molality uses mass, and molarity measures the whole solution where molality measures only the solvent. Making 1 M NaCl means dissolving a mole of salt and topping up with water to exactly 1 L total. Making 1 m NaCl means adding a mole of salt to exactly 1 kg of water — whatever volume that ends up being.
Why temperature is the deciding factor
Heat a solution and it expands: the volume grows, so the molarity drops — even though nothing was added or removed. Mass doesn't care about temperature, so molality stays fixed. That's the entire reason molality exists: for experiments where temperature changes, like boiling-point elevation and freezing-point depression (the chemistry behind salting icy roads), only molality gives consistent numbers.
| Molarity (M) | Molality (m) | |
|---|---|---|
| Formula | mol ÷ L solution | mol ÷ kg solvent |
| Depends on temperature? | yes (volume expands) | no (mass is constant) |
| Easy to prepare with… | volumetric flask | balance |
| Main uses | titrations, dilutions, everyday lab work | colligative properties, precise physical chemistry |
| Unit name | molar | molal |
Worked example: the same solution, both ways
58.44 g of NaCl in 1.00 kg of water
One mole of NaCl (58.44 g) is dissolved in 1.00 kg of water, giving 1.02 L of solution at 20 °C. Find the molality and the molarity.
- Moles of NaCl = 58.44 ÷ 58.44 = 1.00 mol.
- Molality = 1.00 mol ÷ 1.00 kg water = 1.00 m — and it stays 1.00 m at any temperature.
- Molarity = 1.00 mol ÷ 1.02 L = 0.98 M at 20 °C — and it shrinks slightly if the solution warms and expands.
Answer: 1.00 m but 0.98 M — the same solution, two different numbers.
Which one should you use?
- Titrations, dilutions, molar ratios in solution reactions → molarity (use the molarity calculator and dilution calculator).
- Freezing-point depression, boiling-point elevation, osmotic calculations → molality.
- If the problem says “per litre of solution” → molarity; “per kilogram of solvent” → molality. The wording is the giveaway.
Summary
- Molarity M = mol ÷ L of solution; molality m = mol ÷ kg of solvent.
- Molarity changes with temperature (volume expands); molality never does.
- Use molarity for everyday lab work; molality for colligative properties.
- In dilute aqueous solutions the two are numerically similar — don't let that hide the difference.