The molarity formula (M = n / V)
Molarity (M), or molar concentration, is the number of moles of solute dissolved per litre of solution: M = n / V, where n is the number of moles and V is the volume of the solution in litres (L).
Because the number of moles is equal to the mass of solute (m, in grams) divided by its molecular weight or molar mass (MW, in grams per mole): M = m / (MW × V).
Rearranging the formula allows you to solve for any unknown variable: Required mass m = M × V × MW, and Required volume V = m / (M × MW). This relationship is fundamental for preparing accurate standard chemical solutions and biological buffers.
Worked example: Preparing a 1 M NaCl solution
Suppose you want to prepare 500 mL (0.5 L) of a 1.0 M sodium chloride (NaCl) solution. The molecular weight of NaCl is 58.44 g/mol.
Using the mass formula m = M × V × MW: m = 1.0 mol/L × 0.5 L × 58.44 g/mol = 29.22 grams.
Weigh out 29.22 g of pure NaCl, dissolve it in approximately 400 mL of deionised water, and then add water until the total volume reaches exactly 500 mL in a volumetric flask.
Molarity versus Molality
Molarity (M) is temperature-dependent because liquid volume expands and contracts with changes in temperature. It is measured per litre of solution.
Molality (m) measures moles of solute per kilogram of solvent. Because mass does not change with temperature, molality is preferred in precision thermodynamics, such as boiling point elevation and freezing point depression measurements.
Inputs for the molarity calculator
Calculation goal
Choose whether you are calculating the molarity of an existing mixture, the required mass to weigh out, or the volume needed.
Molecular weight (molar mass)
The sum of atomic weights of the chemical formula in g/mol. For example, NaCl is 58.44 g/mol and Glucose is 180.16 g/mol.
Mass and volume
Enter the amount of solute in grams or milligrams, and the volume in litres, millilitres, or microlitres.
CALCULATOR QUESTIONS
Molar concentration calculator questions
What is molarity (M)?
Molarity (M) is the concentration of a solution expressed as the number of moles of solute per litre of solution (1 M = 1 mol/L).
How do I convert molarity (M) to millimolar (mM)?
Multiply by 1,000. For example, a 0.05 M solution is equal to 50 mM, and 1 mM is equal to 1,000 µM (micromolar).
How do you calculate grams from molarity?
Multiply the molarity (mol/L) by the volume in litres (L) and the formula weight (g/mol): Mass (g) = Molarity × Volume × Molecular Weight.
Why does molarity change with temperature?
Because temperature changes cause thermal expansion or contraction of the liquid volume, altering the number of moles per unit volume even though the quantity of solute remains constant.