Molarity is the most common way to express concentration in chemistry. It is the number of moles of solute per liter of solution. The unit is moles per liter, abbreviated M. A 1 M solution of sodium chloride contains one mole of NaCl dissolved in enough water to make one liter of solution. Molarity is temperature-dependent because the volume of a solution changes with temperature.

To prepare a solution of a given molarity, you weigh out the solute, dissolve it in less than the final volume of solvent, and then add solvent until the final volume is reached. The volume is measured after the solute is dissolved, not before. That is because the solute occupies volume and can change the total volume. For precise work, a volumetric flask is used.

Molarity is used in stoichiometry, titration, and reaction kinetics. It allows chemists to calculate how much of a reactant is needed to react with a given volume of solution. The dilution equation, M1V1 = M2V2, relates the molarity and volume of a solution before and after dilution. It is used to prepare working solutions from stock solutions.

Other concentration units exist. Molality is moles of solute per kilogram of solvent, which is temperature-independent. Mass percent is the mass of solute divided by the mass of solution. Parts per million is used for trace concentrations. Each unit has its applications. Molarity is convenient for reactions in solution.