An equivalent (Eq) in chemistry is the amount of a substance that can react with one mole of a counter-ion carrying a unit charge (+1 or -1), such as hydrogen (H+) or hydroxide (OH-). The "equal" part of this term, then, means equal in terms of chemical valence, not in terms of sheer mass.
For example, you may know that one sodium ion (Na+) reacts with one chlorine ion (Cl-) to form sodium chloride or table salt (NaCl). For this reason, any number of Na+ ions will react with an equivalent number of Cl- ions. But one mole (6.022 x 1023 particles) of sodium has a mass of 23.0 grams, whereas one mole of chlorine has a mass of 35.45 grams. Thus equivalents are useful in preparing solutions for specific chemical reactions.
A milliequivalent (mEq) which is 1/1,000th of an equivalent, is a more common measure than equivalents because of the amounts in which substances occur in everyday chemistry situations, which is more often in milligrams than grams.
In contrast to an equivalent, which is a quantity, molarity (M) is concentration, describing the number of moles per liter of a substance in a solution.
One formula for determining milliequivalents is:
mEq = (mg × valence) ÷ molar mass
The information about valence and molar mass is in the periodic table of the elements. Valence is usually evident from the formula of the substance with which you are working. For example, NaCl has a valence of one because Na+ has a charge of +1. CaCl2 has a valence of two because a calcium ion, Ca2+, carries a charge of +2 and requires two negative ions to offset it.
Assume you get 200 mL of a 0.01 M NaCl solution. To calculate the milliequivalents:
Step 1: Determine the Mass of Substance Present
The molar mass of NaCl is the molar mass of the two constituent molecules, Na and Cl, added together. From the periodic table, this is 23.0 + 35.45 = 58.45 g.
Thus a 1 L of a 1 M solution of NaCl would contain 58.45 g. But the molarity in this example (0.01 M) is only 0.01 times this concentration, and the volume is only 0.2 times as much (200 mL/1,000 mL). Therefore, the total mass of NaCal present is:
(58.45 g)(0.01)(0.2) = 0.117 g
Since the problem required milligrams, multiply this by 1,000:
(0.117 g)(1,000 mg/g) = 117 mg
Step 2: Convert Milligrams to Milliequivalents
Using the formula above, mEq = (mg × valence) ÷ molar mass, gives
mEq = (117 mg × 1) ÷ 58.45 = 2 mEq