concentration, in chemistry, measure of the relative proportions of two or more quantities in a mixture. The concentration of a solute is very important in studying chemical reactions because it determines how often molecules collide in solution and thus indirectly determines the rates of reactions and the conditions at equilibrium (see chemical equilibrium).
Concentration may be expressed in a number of ways. The simplest statement of the concentrations of the components of a mixture is in terms of their percentages by weight or volume. Mixtures of solids or liquids are frequently specified by weight percentage concentrations, such as alloys of metals or mixtures used in cooking, whereas mixtures of gases are usually specified by volume percentages. Very low concentrations may be expressed in parts per million (ppm), as in specifying the relative presence of various substances in the atmosphere.
In addition to these means of expressing concentration, several others are defined especially for describing solutions: molarity, molality, mole fraction, formality, and normality. Some of these define the concentration of the solute in reference to the amount of solvent, others in reference to the total amount of solution. The molarity of a solution is the number of moles of solute per liter of solution; e.g., a solution of glucose in water containing 180.16 grams (1 gram-molecular weight, or mole) of glucose per liter of solution is referred to as one molar (1 M ). The molality of a solution is the number of moles of solute per 1,000 grams of solvent; a solution prepared by dissolving 180.16 grams of glucose in 1,000 grams of water is one molal (1 m ). The mole fraction of a solution is the ratio of moles of solute to the total number of moles in the solution. Since ionic compounds, such as sodium chloride, NaCl, do not occur as molecules, their concentrations cannot be expressed in terms of molarity, molality, or mole fraction. Instead, the concentration of an ionic compound in solution may be given by its formality, the number of gram-formula weights of the compound per liter of solution; e.g., a solution containing 58.44 grams (one gram-formula weight) of NaCl per liter of solution is one formal (1 F ). In considering the reactions of certain solutions in combination, for example the neutralization of acids and bases, a useful expression of the concentration is the normality of each solution, the number of gram-equivalent weights of solute per liter of solution (see equivalent weight); e.g., a solution containing 49.04 grams (one gram-equivalent weight) of sulfuric acid, H2SO4, per liter of solution is one normal (1 N ). Concentrations of solutions may also frequently be given in terms of the weight of solute in a given volume of solvent or solution.