Concentration scales

(chemistry) Any of several numerical systems defining the quantitative relation of the components of a mixture; for solutions, concentration is expressed as the mass, volume, or number of moles of solute present in proportion to the amount of solvent or total solution.

Concentration is a very important property of mixtures, because it defines the quantitative relation of the components. In solutions the concentration is expressed as the mass, volume, or number of moles of solute present in proportion to the amount of solvent or of total solution.

The simplest scale to measure is percentage; hence it is often used for medicinal or household solutions. Weight percent is the number of parts of weight of solute per hundred parts of solution (total). For example, a 10% saline solution contains 10 g of salt in 90 g of water, that is, 100 g total weight. Gaseous mixtures, being difficult to weigh, are often expressed as volume percent. Thus, air is said to contain 78% nitrogen by volume. Solutions of liquids in liquids (say, alcohol in water) may also be expressed in volume percent.

To the chemist, the number of moles of solute is of more significance than the number of grams. The molarity (abbreviated M) is the number of moles of solute per liter of total solution. Thus, 12 M HCl means that the solution contains 12 formula weights (12 × 36.5), or 438 g, of HCl/liter. (As used here, the mole is an amount of substance whose weight in grams is numerically the same as the molecular weight.)

Molality (abbreviated m) relates the number of moles of solute to the weight of solvent rather than to the volume of solution. This scale indicates the number of moles of solute/1000 g of solvent. Thus 34.2 g of sucrose (C₁₂H₂₂O₁₁, mol wt 342), if dissolved in 200 g of water, has the concentration of 0.5 mole of sucrose/1000 g of water, and hence is 0.5 m.

When it is important to know the reactive capacities of reagents, as in volumetric analysis, the normality scale is used. Normality (abbreviated N) is found by multiplying molarity by the number of active units in the formula.

In recent years many chemists have sought to avoid the molarity scale lest it imply that electrolyte solutes exist as molecules rather than ions. The formality scale (abbreviated F) represents formula weights per liter. Its values are identical with the molarities of un-ionized solutes.

Many properties of solutions (for example, vapor pressure of one component) are dependent on the ratio of the number of moles of solute to the number of moles of solvent, rather than on the ratios of respective volumes or masses. The mole fraction (abbreviated N_A or X_A for component A) is the ratio of the number of moles of solute to the total number of moles of all components. Thus for 16 g of methanol (0.5 mole) dissolved in 18 g of water (1 mole), the mole fraction of methanol is 0.5/1.5, or 1/3; the mole percent is 33.3. For gases the mole percent is identical with the volume percent. See also Gram-molecular weight; Solution; Titration.