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Equivalent (chemistry)

    The '''equivalent''' (Eq or eq) is a reasonably common measurement unit used in chemistry and the biology|biological sciences It is a measure of a chemical compound|substance's ability to combine with other substances It is frequently used in the context of normality
    The equivalent is formally defined as the mass in grams of a substance which will react with 6022 x 1023 electrons (This is Avogadro's Number which is the number of particles in a mole)
    Another slightly less precise definition describes the equivalent as the number of grams of a substance that will react with a gram of free hydrogen (This is practically true since a gram of hydrogen is very close to a mole of hydrogen and free hydrogen has one spare electron; hence one gram of hydrogen is effectively equivalent to 6022 x 1023 electrons)
    Hence the equivalent weight of a given substance is effectively equal to the amount of substance in moles divided by the valence of the substance
    In practise the equivalent weight often has a very small magnitude so it is frequently described in terms of milliequivalents (mEq or meq) -- the prefix milli denoting that the measure is divided by 1000 Very often the measure is used in terms of milliequivalents of solute per litre of solvent (mEq/L) This is especially common for measurement of compounds in biological fluids; for instance the healthy level of potassium in the blood of a human is defined between 3.5 and 5.0 mEq/L
    Equivalents have advantage over other measures of concentration (such as moles) in quantitative analysis of reactions The best feature of using equivalents is that there is no need to study much about the nature of reaction ie no need to analyse and balancing chemical equations Equivalents of reactants react in equal numbers to yield the equal equivalents of products
    So in case of "weird" chemical equations (?) equivalents (or milliequivalents) are used

    Conversion guide

    For monovalent ions 1 equivalent = 1 mole
    For divalent ions 1 Eq = 0.5 mol
    For trivalent ions 1 Eq = 0333 mol

    Sources

    A dictionary of units of measurement
    Converting moles to milliequivalents