Molarity Calculator

How the molarity calculator works

Molarity (M) is the concentration of a solution: moles of solute per litre of solution:

Molarity (mol/L) = moles of solute ÷ volume of solution (L)

Use the final volume of the solution, not the volume of water you started with.

Worked examples

1. 0.5 mol of NaCl dissolved to make 250 mL (0.25 L) of solution: 0.5 ÷ 0.25 = 2.0 M
2. From a mass: 5.844 g of NaCl (molar mass 58.44 g/mol) is 0.1 mol. Made up to 500 mL: 0.1 ÷ 0.5 = 0.2 M

Working from grams

Most lab work starts with a mass, so find the moles first:

Moles = mass (g) ÷ molar mass (g/mol)

Then divide by the volume in litres. Our moles from mass calculator does the first step.

Making a solution in the lab

1. Weigh the solute accurately.
2. Dissolve it in less water than the final volume.
3. Transfer to a volumetric flask and make up to the mark with water.
4. Mix thoroughly.

Diluting a stock solution

Use C₁V₁ = C₂V₂. To make 100 mL of 0.2 M from a 2.0 M stock: V₁ = 0.2 × 100 ÷ 2.0 = 10 mL of stock, made up to 100 mL. The dilution calculator handles this.

Molarity vs molality

• Molarity is moles per litre of solution. It changes slightly with temperature because volume does.
• Molality is moles per kilogram of solvent. It doesn't change with temperature, which is why it's used in colligative property calculations.

How do I calculate molarity?

Divide moles of solute by litres of solution. 0.25 mol in 0.5 L is 0.5 M.

How do I convert millilitres to litres for molarity?

Divide by 1,000. For example, 250 mL = 0.25 L.

What is a 1 M solution?

One mole of solute per litre of solution. For NaCl, that's 58.44 g made up to 1 L.

Written by Sam Alex, civil engineer. About the author