The normal concentration is another method for expressing the concentration of solutions. Normality (N) is defined as the number of equivalents of solute dissolved in one liter of solution.

One equivalent of acid is the amount of acid necessary to give up one mole of hydrogen ions in a chemical reaction. One equivalent of base is the amount of base that reacts with one mole of hydrogen ions. When expressing the concentrations of bases, normality refers to the number of available hydroxyl ions. Because hydrogen and hydroxyl ions combine on a one-to-one basis, one OR is equivalent to one H+ ion.

Notice that in reaction a), one mole of HCl yields one equivalent per mole or one mole of H+ ions. H₂SO₄ has two equivalents per mole or two H+ ions because each mole of the compound can release two moles of hydrogen ions.

The number of equivalents of an acid or base can be determined from equivalent weight. The equivalent weight is defined as the molecular weight of the acid or base divided by the number of replaceable hydrogen or hydroxyl ions.

Example:

This means 49 grams of H₂SO₄ is equal to one equivalent of H₂SO_4. If there is a one

liter solution that contains 78.32 grams H₂SO₄, the number of equivalents is:

Since normality is equal to the number of equivalents per liter, the normality of this

solution is 1.6 equivalents/liter, or 1.6 N.

Parts per Million

Another term used to describe the specific concentration of a solution is parts per million or ppm. The term ppm is defined as the concentration of a solution in units of one part of solute to one million parts solvent. One ppm equals one milligram of solute per liter of solution. Another term, parts per billion (ppb), is defined as one part solute per one billion parts solvent. One ppb is equal to one microgram solute per liter of solution. These two terms are usually used for very dilute solutions.