From Moles to Milliequivalents

Many important substances in the body are measured in equivalents. The technical definition of an equivalent is the amount of substance it takes to combine with 1 mole of hydrogen ions. As an example, we can look at hydrochloric acid (HCl). It takes approximately 35 grams of chloride (1 mole), to combine with 1 gram of hydrogen (1 mole) to make 1 mole of HCl (which weighs approximately 36 grams). Since both of these elements are monovalent (carrying a valence charge with a magnitude of 1), they combine in a one-to-one ratio. Therefore the amount of chloride that is needed to combine with one mole of hydrogen is 1 mole or 1 equilvalent (eq).

As another example, let’s say you have a divalent ion (Ca+2) combining with Cl- (a monovalent ion) to form calcium chloride (CaCl2). The combining ratio of calcium to chlorine is one-to-two, meaning it takes half a mole of Ca+2 to combine with one mole (or 1 eq) of Cl-. So, half a mole of calcium equals 1 equivalent.

Another way to look at it is to say one equivalent is the amount of ion required to cancel out the electrical charge of an oppositely charged monovalent ion. You can also say that the valence charge of the ion is the number of equivalents there are in one mole of that ion. For example, nitrogen (N-3) is a trivalent ion; therefore one mole of nitrogen equals 3 equivalents. Using the calcium example from above, we know that one mole of calcium equals 2 equivalents.

Equivalents can also be defined in terms of metric weight. The weight of one equivalent can be determined by dividing the gram-atomic weight of the ion by its valence. For calcium, the equation would look like this:



We know that calcium weighs approximately 40 g. Since 1 equivalent of calcium is half a mole, we can see that 1 equivalent weighs approximately 20 g.

Remember

Most solutes in the body are not measured in grams and moles, but milligrams and millimoles instead. So we can use the term milliequivalent when discussing such substances. Here are some things to keep in mind when converting to milliequivalents:

1 meq = 10-3 eq

For monovalent ions, 1 meq = 1 mmol

For divalent ions, 1 meq = 0.5 mmol

For trivalent ions, 1 meq » 0.333 mmol

Exercises
1. How many equivalents are present in 80 grams of calcium (molecular weight » 40 g)?
2. How many equivalents of sodium are present in 116 g of NaCl (molecular weight of Na » 23; molecular weight of Cl » 35)?
3. How many millimoles of Mg+2 would be present in a solution containing 0.8 milliequivalents?
4. How many milliequivalents of P-3 are present in a solution containing 6 millimoles?