Molarity and solution concentration calculator

Molarity is the number of moles of solute per litre of finished solution. It is the working unit of solution chemistry because reactions consume particles in whole-number ratios, and moles count particles.

Here c is concentration in mol/L, n is moles, V is volume in litres, m is mass in grams and M is molar mass in g/mol.

Preparing a solution

To make 500 mL of 0.100 M sodium chloride: you need 0.100 × 0.500 = 0.0500 mol. NaCl has a molar mass of 58.44 g/mol, so weigh out 0.0500 × 58.44 = 2.92 g.

The step people get wrong is the last one. Dissolve the salt in less water than the target volume, then top up to the 500 mL mark in a volumetric flask. Do not add 500 mL of water to the salt — the solute takes up volume of its own, so you would end up with more than 500 mL of solution and a concentration below what you wanted.

Molarity, molality and normality

• Molarity (M, mol/L) is per litre of solution. It changes with temperature because the solution expands.
• Molality (m, mol/kg) is per kilogram of solvent. Temperature has no effect on mass, so molality is what you use for colligative properties like boiling point elevation.
• Normality (N) is molarity multiplied by the number of reactive equivalents. For 1 M sulfuric acid, which supplies two protons, the normality is 2 N. It survives mainly in titration work.

Dilution

Diluting a stock solution does not change the number of moles present, only the volume they occupy. That gives the dilution equation:

To make 250 mL of 0.10 M from a 2.0 M stock: V₁ = (0.10 × 0.250) / 2.0 = 0.0125 L, so take 12.5 mL of stock and dilute to 250 mL. With concentrated acids, add the acid to the water and never the reverse — the dilution is strongly exothermic and adding water to acid can boil it out of the container.

Molar masses worth memorising