How to use
- Enter three of the four values — mass (g), molecular weight (g/mol), volume and molarity (M).
- Volume can be given in mL or L.
- The remaining value is calculated for you.
- The molecular weight is on the reagent label or the product datasheet. For a hydrate, use the value that includes the water of crystallisation.
Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula & notes
Molarity (M) is the amount of substance dissolved in one litre of solution. That amount is the mass divided by the molecular weight, which gives the relationship between the three quantities.
- Amount of substance
- mol = mass (g) ÷ MW (g/mol)
- Molarity
- M (mol/L) = mol ÷ volume (L)
Put the two together and you have the single equation this calculator uses.
- Mass
- mass (g) = MW (g/mol) × M (mol/L) × volume (L)
Knowing three of the four values, the fourth comes from rearranging it. A volume entered in mL is converted to litres before the calculation.
Practical notes
- The volume is the final volume of the solution, not the amount of solvent. You are not dissolving the solid in 1 L of water; you are dissolving it and then making the whole up to 1 L. A dissolved solid adds volume of its own, so the two routes give different concentrations.
- For a hydrate, use the molecular weight that includes the water of crystallisation. CuSO₄ (159.6) and CuSO₄·5H₂O (249.7) are different numbers, and missing the difference puts the concentration out by more than half again.
- A reagent below 100% purity needs its weighed mass corrected: divide by the purity. At 95% purity, weigh out the calculated mass ÷ 0.95.
- Hygroscopic reagents take up moisture from the air and their weight drifts while the balance is open. Weigh quickly and close the bottle.
- For a poorly soluble substance the calculated concentration and the dissolved concentration are not the same thing. Check that the target is below the solubility first.