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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.

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