The formula
Molarity (M) is moles of solute per liter 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
- Weigh out the solute.
- Dissolve it in less water than the final volume, about 80%.
- 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 milliliters to liters: 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.
A worked example with a hydrate
For 100 mL of 0.5 M copper sulfate from the blue crystals, CuSO₄·5H₂O (249.68 g/mol):
0.5 × 0.1 × 249.68 = 12.48 g.
Using the molar mass of anhydrous CuSO₄ (159.61 g/mol) instead gives 7.98 g, and the solution comes out about 36% weaker than intended. The Molar Mass Calculator takes the dot in hydrate formulas.
Molar, percent and mg/mL
1 M sodium chloride is 58.44 g per liter, which is the same as 58.44 mg/mL or 5.844% weight per volume. Multiply molarity by molar mass to get g/L, and divide g/L by 10 to get percent.
Common problems
- The solid will not dissolve. Some salts and buffers dissolve slowly or only near a certain pH. Stir, warm gently if the compound allows it, and adjust the pH before bringing it to volume.
- The volume shrinks or grows. Ethanol and water, or concentrated acids and water, do not add up to the sum of their volumes. Always make up to the final volume rather than adding a calculated amount of water.