How to Calculate Equivalent Units

By Timothy Banas
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Chemists use equivalent units, or equivalents, to express the contribution of an acid or base to the total acidity or alkalinity of a solution. To calculate the pH of a solution--the measure of a solution's acidity--you need to know how many hydrogen ions are present in the solution. The most common way of determining this is by measuring the amount of acid that you have added to the solution. But different acids contribute different numbers of hydrogen ions to a solution. For instance, hydrochloric acid (HCL) contributes 1 ion per molecule of acid, but sulfuric acid (H2SO3) contributes 2 ions per molecule of acid. Therefore, it is said that adding 1 molecule of HCL is 'equivalent' to adding 1 ion, but adding 1 molecule of H2SO4 is 'equivalent' to adding 2 ions. This creates the need for the "equivalent unit."

Consider the chemical formula of the acid you are using. The most common strong acids and their formulas are:

Hydrochloric: HCL Sufuric: H2SO4 Phosphoric: H3PO4 Nitric: HNO3 Hydrobromic: HBr Hydroiodic: HI Perchloric: HCLO4 Chloric: HClO3

Determine the equivalents contained in 1 mole of each acid by looking at the number directly after the H in the chemical formula of each acid. If there is no number directly after the H, the number is assumed to be 1. The number of equivalents per mole of acid is equal to that number. For instance, sulfuric acid has a molar equivalent of 2 because there is a 2 after the H in its formula.

Determine the number of moles of acid you have added to a solution by multiplying its molarity (M) by the volume you added. For example, suppose you added 0.3 liters (L) of 0.5 M sulfuric acid to a solution. The number of moles you added would be:

Number of moles = 0.3 x 0.5 = 0.15 moles of sulfuric acid

Calculate the number of acid equivalents you added to the solution by multiplying the number of moles you added by the equivalents associated with each molecule of that acid. Because sulfuric acid yields 2 equivalents per mole:

Equivalents = 0.15 moles x 2 equivalents/mole = 0.3 equivalents

In our example, you added 0.3 molar equivalents of acid to the solution.

About the Author

Timothy Banas has a master's degree in biophysics and was a high school science teacher in Chicago for seven years. He has since been working as a trading systems analyst, standardized test item developer, and freelance writer. As a freelancer, he has written articles on everything from personal finances to computer technology.