The equivalent (Eq or eq) is a reasonably common measurement unit used in chemistry and the biological sciences. It is a measure of a 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 6.022 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 6.022 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 is often a very large unit, 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, i.e. 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 = 0.333 mol

Sources

A dictionary of units of measurement

Converting moles to milliequivalents