How to Make a Solution of a Given Molarity

The formula

Molarity (M) is moles of solute per litre of solution. The mass to weigh out is:

mass (g) = molarity (mol/L) × volume (L) × molar mass (g/mol)

For 500 mL of 0.1 M sodium chloride (58.44 g/mol): 0.1 × 0.5 × 58.44 = 2.922 g.

Making it up

1. Weigh out the solute.
2. Dissolve it in less water than the final volume, about 80%.
3. Transfer it to a volumetric flask or measuring cylinder and top up to the final volume.

Adding the solid to the full volume of water gives a slightly lower concentration, because the dissolved solid takes up space too.

Common slips

• Convert millilitres to litres: 250 mL is 0.25 L.
• Convert mM to M: 50 mM is 0.05 M.
• Use the molar mass of the form in the bottle, including water in a hydrate.

Skip the arithmetic

The Molarity Calculator works out the mass, molarity or volume, and takes a formula instead of a molar mass if you prefer. To switch between M and mg/mL or ppm, use the Concentration Converter.