Online Introductory Chemistry
Equivalents & milliequivalents
 

Equivalents and milliequivalents

Equivalent An equivalent is an amount of material that will release or react with an Avogadro's number of electrical charges on particles like OH1-, H1+ or electrons.

This means that an equivalent of HCl will react with a mol of hydroxide ion charge.

For bases this means that an equivalent of KOH will react with a mol of hydrogen ion charge.

In an oxidation reaction a metal like zinc, Zn, can be oxidized. A mol of zinc will oxidize to produce Zn2+ and release two mols of electrons. Zn0 ---> Zn2+ + 2e1-

In reduction a nonmetal like sulfur can accept two electrons to form sulfide ions, S2-. One mol of sulfur will equal two equivalents in this reduction half reaction. S0 + 2e1- ---> S 2-

Usually you can count the number of acidic hydrogens in an acid and decide how many equivalents there are in a mol. For example HF (hydrofluoric acid) will have 1 equivalent per mol while H2SO4 (sulfuric acid) will have 2 equivalents per mol of H2SO4.

Bases like Al(OH)3 can be analyzed in a similar way. You can count the number of hydroxide ions in the formula to get the number of equivalents per mol. Aluminum hydroxide, has three hydoxide ions Al(OH)3

Example 1

1 mol Ca(OH)2 = 2 equivalents Ca(OH)2

There are "TWO" OH1- released when the hydroxide dissolves.

The calcium hydroxide releases Ca2+ and two OH1- ions.

The equivalent weight of calcium hydroxide is 1/2 he mass of a mol of calcium hydroxide.

1 mol Ca(OH)2 = 74 grams Ca(OH)2 ; 1 equivalent Ca(OH)2 = 37 grams Ca(OH)2

Example 2

1 mol Al(OH)3= 3 equivalents Al(OH)3

There are "THREE" OH1- released when the hydroxide dissolves.

The calcium hydroxide releases one Al3+ and three OH1- ions.

The equivalent weight of aluminum hydroxide is 1/3 the mass of a mol of aluminum hydroxide.

1 mol Al(OH)3 = 78 grams Al(OH)3 ; 1 equivalent Al(OH)3 = 26 grams Al(OH)3

Example 3

1 mol H2SO4 = 2 equivalents H2SO4

There are "TWO" H1+ released when the acid dissociates

The acid releases two H1+ ions and one SO42- ion.

The equivalent weight of sulfuric acid is 1/2 the mass of a mol.

1 mol H2SO4 = 98 grams H2SO4 ; 1 equivalent H2SO4 = 49 grams H2SO4

Example 4

1 mol H3PO4 = 3 equivalents H3PO4

There are "THREE" H1+ released when the acid dissociates

The acid releases three H1+ and one phosphate PO42- ion.

The equivalent weight of phosphoric acid is 1/3 the mass of a mol.

1 mol H2PO4 = 98 grams H3PO4 ; 1 equivalent H3PO4 = 32 grams H3PO4

Milliequivalents A milliequivalent is 1/1000 of a equivalent. This means an equivalent is 1000 milliequivalents. In terms of grams the equivalent weight is equal to 1000 milliequivalent weights. THIS UNIT IS USEFUL because it matches the levels of dissolved ions in small volumes of body fluids.

Example 1

1 mol H3PO4 = 3 equivalents H3PO4

The acid releases three H1+ and one phosphate PO42- ion.

The equivalent weight of phosphoric acid is 1/3 the mass of a mol.

1 mol H2PO4 = 98 grams H3PO4 ; 1 equivalent H3PO4 = 32 grams H3PO4

The relation between equivalents and milliequivalents

1 equivalent H3PO4 = 1000 milliequivalent H3PO4

1 equivalent mol H2PO4 = 32 grams H3PO4 ;

1 milliequivalent mol H2PO4 = 32 milligrams H3PO4 ;

HERE THE TWO SIDES OF THE EQUATION HAVE BEEN DIVIDED BY 1000

Online Introductory Chemistry

Dr. Walt Volland all rights reserved 1997-2005, revised March 29, 2005