Short answer
Molarity is moles of solute divided by liters of the entire solution. Molality is moles of solute divided by kilograms of solvent alone. Their denominators differ, so a 1 M solution is not generally 1 m. 1 2
Both count moles of solute, but molarity uses solution volume while molality uses solvent mass.
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At a glance
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| Question | Molarity | Molality |
|---|---|---|
| Formula | mol solute / L solution | mol solute / kg solvent |
| Symbol | M | m |
| Temperature sensitivity | Volume can change | Mass basis is stable |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Molarity. Moles of solute per liter of total solution, symbol M. 1
Molality. Moles of solute per kilogram of solvent, symbol m. 2
Key differences
The solution volume includes solute and solvent; solvent mass excludes solute. Converting between units therefore requires density and composition information, not a simple replacement of liters with kilograms. 1 2
How to tell them apart
Read the denominator on a label or calculation. “Per liter of solution” means molarity; “per kilogram of solvent” means molality. Keep capital M and lowercase m distinct. 1 2
Where they overlap
Both are concentration units used to describe dissolved material and can be calculated from the same prepared sample if enough measurements are known. 1 2
Edge cases
At very low concentrations in water, numerical values may be close but not identical. Temperature changes solution volume, while solvent mass stays essentially fixed, making molality useful for colligative properties. 1 2
Why the distinction exists
Different experiments need different denominators: measured solution volume for many reactions, solvent mass for temperature-sensitive solution properties. 1 2
Common misconceptions
Molality is not “moles per kilogram of solution”; its denominator is kilograms of solvent. 1 2
Examples
One mole of glucose in one liter of final solution is 1 M. One mole in one kilogram of water is 1 m, but the final solution volume need not be one liter. 1 2
Sources
Sources checked October 3, 2026.
- OpenStax — Molarity. Concentration and molarity.
- OpenStax — Colligative Properties. Mole fraction and molality.