Molarity measures the concentration of a solution — how much solute (in moles) is dissolved per liter of solution. It's one of the most common concentration units in chemistry, used for everything from lab reagent prep to titration calculations.
The formula
For 0.5 moles of a solute dissolved to make 2 liters of solution: molarity = 0.5 ÷ 2 = 0.25 mol/L (also written as 0.25 M).
Worked examples
| Moles of solute | Volume | Molarity |
|---|---|---|
| 1 mol | 1 L | 1.0 M |
| 0.1 mol | 0.5 L | 0.2 M |
| 2 mol | 4 L | 0.5 M |
Important distinction: solution volume, not solvent volume
Molarity is defined using the volume of the final solution (solute plus solvent combined), not just the volume of solvent added. If you dissolve a solid solute into a liquid, the final solution volume may differ slightly from the starting solvent volume, especially at higher concentrations — for precise lab work, the solution is typically made up to a target final volume using a volumetric flask.
Common mistakes
- Using solvent volume instead of solution volume. This is a very common error in introductory chemistry that leads to a slightly inaccurate molarity, particularly for concentrated solutions.
- Confusing molarity with molality. Molarity is moles per liter of solution; molality is moles per kilogram of solvent — a related but different concentration measure, more common in contexts where temperature-dependent volume changes matter.
Frequently asked questions
How do I calculate molarity?
Divide the number of moles of solute by the total volume of the solution in liters.
Is molarity the same as concentration?
Molarity is one specific way of expressing concentration (moles per liter of solution). Other concentration units exist, such as molality (moles per kilogram of solvent) or mass percent, used in different contexts.
Should I use the volume of solvent or the final solution?
Use the volume of the final solution after the solute is fully dissolved, not just the starting solvent volume, since these can differ.