Concentration





Concentration is a measure of the amount of one specified element or
component in a compound or mixture. Concentration may be expressed both
qualitatively or quantitatively.

Qualitatively, terms such as dilute and concentrated are use to describe
relatively low and high concentrations, respectively. Quantitatively, there
are a number of different ways to express the ratio of the part to the
whole. The most common are:

   * Mass per unit mass - percentage by mass. For example, if substance a
     has a mass of 50g in a total mixture mass of 100g, then substance A has
     a concentration of 50% m/m in that mixture.

   * Molarity (M), or moles per liter. For example, a 1.0 liter solution
     containing 5.0 moles of a solute would have a concentration of 5.0 M.
     Although molarity is the most common method of specifying
     concentration, it is dependent on temperature and pressure (due to
     change in volume). If an experimenting is being performed such that the
     charge of a solution is important, then knowing the mass proportion of
     the ions in the solution is irrelevant, as different ions will have
     different masses, but may have the same charge. Thus, the molar
     concentration of the substance is required, as molarity takes into
     account the atomic mass of the substance.

   * Molality (m), or moles per kilogram. For example, a 1.0 kg solution
     containing 5.0 moles of solute would have a concentration of 5.0 m.
     Molality is used when temperature varies in an exothermic or
     endothermic reaction because it is not dependent on temperature or
     pressure. The unit symbol for molality, m, which conflicts with the SI
     unit meter; the intended unit can be determined by context.

   * Mole fraction (χ), or moles per moles. For example, 5.0 moles
     dissolved in a 10 mole solution would have a mole fraction
     concentration of 2.0. Since the number of moles essentially measures
     particles, the mole fraction measures particles per particles.

   * For extremely low concentration, the following units are used:
        o Parts per million (ppm)
        o Parts per billion (ppb)
        o Parts per trillion (ppt)