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Solutions are homogeneous mixtures that result from dissolving, at the molecular level, one or more "solutes" within the "solvent" — the dissolving medium. The solvent also may consist of more than one substance, as long as they, too, dissolve in one another. In ordinary usage, the word solution refers to substances dissolved in a liquid solvent, although the broad use of the word is not so limited. Scientists call solute quantity or richness within a solution its solution concentration. There are a number of ways to quantify — or assign a numerical value to — solution concentration.
Methods used to report concentration vary, depending upon whether the use is scientific or not — and sometimes depending upon the particular science. Especially among analytical chemists, the most common unit for reporting solution concentration is "molarity." This term is derived from the word "mole," referring to a particular compound's molecular weight in grams. It can be readily seen that, since their molecular weights differ, a mole of sugar is not equal in weight to a mole of salt.
Consider how a one molar solution of table salt is prepared. Sodium chloride has the chemical formula NaCl — it is the reaction product produced by combining sodium (Na) metal with chlorine (Cl) gas. The atomic weight of sodium is 22.99; chlorine’s atomic weight is 35.45. Simple addition gives the molecular weight of salt as 58.44 — that is, a mole of NaCl weighs 58.44 grams. Dissolving this quantity of NaCl in water to make a liter (1.06 quarts) of solution results in an exactly one molar solution (1.0 M).
Less frequently, solution concentration may be expressed in terms of "normality" or "molality." The definition for normality is not much different from that for molarity, but incorporates the concept of "equivalents." As an example, a solution that is 1.0 molar in phosphoric acid (H3PO4), since it produces three hydrogen ions for every molecule of phosphoric acid, is 3.0 normal (3.0 N). Although at first it might seem advantageous to use normality in place of molarity as the standard of solution concentration, normality is not an absolute term, but depends upon the use of the solution. For this reason, the International Union of Pure and Applied Chemistry has urged the discontinuance of normality in expressing solution concentration.
Molality is used even less often than normality is. A solution is one molal (1.0 m) if it consists of one mole of solute dissolved in a kilogram — not of solution — but of solvent. At first, it might seem that molality affords no especially valuable properties making it useful as a term for solution concentration. It does not involve volume, however, but only weight — for both solute and solvent. This means molality is not temperature-dependent, making it the unit of choice in those areas of chemistry involving "colligative" properties — those properties involving numbers of particles.
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