How to Calculate Molarity Step by Step

Molarity (symbol M) expresses concentration as moles of solute dissolved in one liter of total solution. It is the most common concentration unit in general chemistry labs because it pairs directly with stoichiometry — you can read mole ratios from a balanced equation and know exactly how many moles of reagent a given volume of solution contains.

The molarity formula

Molarity equals moles of solute divided by volume of solution in liters: M = n / V. If you dissolve 0.50 mol of NaCl in enough water to make 2.0 L of solution, the molarity is 0.50 / 2.0 = 0.25 M. Note that volume refers to the final solution volume, not the volume of water added.

Preparing a solution from solid solute

1. Calculate moles needed: n = M × V (volume in liters)
2. Convert moles to mass: mass = n × molar mass
3. Weigh the solid and transfer to a volumetric flask
4. Add solvent partially, dissolve completely, then fill to the calibration mark
5. Mix thoroughly before use

Dilution with C₁V₁ = C₂V₂

When you dilute a stock solution, the number of moles of solute stays constant — only the volume changes. The dilution equation C₁V₁ = C₂V₂ relates initial concentration and volume to final concentration and volume. Always use consistent units for volume (mL or L) on both sides.

Molarity vs molality and other units

Molality (m) uses kilograms of solvent instead of liters of solution, making it temperature-independent — important for colligative property problems. Mass percent, ppm, and normality appear in industry and specialized labs. The Solution Concentration Calculator converts between common units when you need to compare scales.

Worked example

How many grams of NaOH (molar mass 40.0 g/mol) are needed to prepare 500 mL of 0.200 M solution? Moles needed = 0.200 × 0.500 = 0.100 mol. Mass = 0.100 × 40.0 = 4.00 g. Dissolve 4.00 g NaOH and dilute to 500 mL in a volumetric flask.

Frequently asked questions