Molarity calculator
Convert between mass, molar concentration and volume for a solution.
From the reagent bottle or supplier datasheet.
Mass required
That solution is 58.44 mg/mL.
m = c × V × M- Definition of amount-of-substance concentration (molarity) — IUPAC Compendium of Chemical Terminology (the Gold Book), 2019
When to use this
Use this to get from a concentration you want to a mass on the balance, or to work backwards from a mass you have already weighed. It needs a molar mass, so it is for defined compounds — for a protein or an extract quoted in mg/mL, the mass-concentration route in the dilution calculator is the right one instead.
Worked example
You need 500 mL of 50 mM Tris for a buffer.
- Solve for
- Mass
- Molar mass
- 121.14 g/mol (Tris base)
- Concentration
- 50 mM
- Volume
- 500 mL
Result
Weigh 3.034 g, dissolve in about 400 mL, adjust the pH, then make up to 500 mL — not the other way round.
What people get wrong
- Taking the molar mass from the compound name rather than the bottle. Tris base is 121.14 g/mol and Tris·HCl is 157.6; the hydrate on the label decides the number, not the name of the chemical.
- Dissolving into the full final volume. Solids take up space, so a solid dissolved in 500 mL of water gives more than 500 mL of solution and a concentration below target.
- Ignoring purity. A reagent quoted at 95% needs proportionally more mass, and the certificate of analysis is where that number lives.
Questions
+Where do I find the molar mass?
On the bottle, or from the formula using the molecular weight calculator, which parses hydrates such as CuSO4·5H2O correctly.
+Why does my answer differ from a supplier’s calculator in the last digit?
Molar masses here follow IUPAC’s current abridged atomic weights. Suppliers sometimes use older values, and for elements such as sulfur the standard atomic weight is an interval rather than a single number.
+Can I use this for a protein?
Only if you know its molar mass, which the peptide analyser will compute from a sequence. In practice protein stocks are quoted in mg/mL, so the mass-concentration units are usually easier.
Science last reviewed .