Molarity Calculator

How to use this calculator

To prepare a solution, enter the molar mass of the compound, the target molarity and the volume; the mass to weigh appears in the first field. To check a solution, enter the mass you dissolved, the molar mass and the final volume to get the concentration. The middle field, moles of solute, connects the two steps and can be used on its own when you already know the amount in moles.

Molar masses are on the container label or can be summed from atomic weights (NaCl: 22.99 + 35.45 = 58.44 g/mol). For hydrates, use the molar mass of the hydrated form you actually weigh, e.g. CuSO₄·5H₂O = 249.69 g/mol.

The formula

n = m / M

c = n / V

m: mass of solute
M: molar mass
n: amount of solute
V: solution volume
c: molarity

Worked example

5.844 g of sodium chloride (molar mass 58.44 g/mol) is dissolved to make 500 mL of solution.

n = m / M = 5.844 / 58.44 = 0.1 mol.
c = n / V = 0.1 mol / 0.5 L = 0.2 M.

The solution is 0.2 mol/L (200 mM) NaCl.

Assumptions and limits

V is the final volume of solution, not the volume of solvent added.
The solute is fully dissolved and pure; adjust the mass for purity (divide by 0.98 for a 98% reagent).

Frequently asked questions

What is the difference between molarity and molality?

Molarity is moles per liter of solution; molality is moles per kilogram of solvent. Molality does not change with temperature, molarity does slightly.

How do I convert mg/mL to molarity?

Divide mg/mL by molar mass in g/mol: the result is mol/L. 5.844 mg/mL NaCl ÷ 58.44 = 0.1 M.

What does mM mean?

Millimolar: one thousandth of a mole per liter. 0.2 M = 200 mM = 200,000 µM.