Equivalent Weight: Definition & Formula

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• 0:58 The Individual Elements
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Lesson Transcript
Instructor: Matthew Bergstresser
In chemistry, there are different ways to express the weights of different compounds. In this lesson, we will learn how to determine the equivalent weight of a compound.

Equivalent Weight

Equivalent weight isn't really a weight-- it's a mass. Mass is the amount of matter in something, while weight is the gravitational pull on a mass. Since we're only dealing with different chemical compounds on Earth, and we're dealing with chemistry, it's acceptable to interchange weight and mass. We'll use the term equivalent weight (EW) for this lesson, and it has a general formula of

Molar mass is the mass of an individual element from the periodic table or, for a compound, the sum of all of the atomic masses in the compound. The n number of equivalents in this part of this equation is the quantity of charge in an equation, and what charge is used depends on the scenario.

Let's look at a few scenarios where the n number of equivalents means different things.

The Individual Elements

The n number of equivalents in the equivalent weight formula for an individual element is the absolute value of that element's valence charge. The valence charge of a main group element in families 1 through 4 is simply its family number. For example, lithium is in family 1. Therefore, its valence charge is 1. For a main group element in families 5 through 8, you subtract 8 from its family number. For example, nitrogen is in family 5, and its valence charge is 5 - 8 which is -3.

Here's an example we can use. We'll begin with the prompt: What is the equivalent weight of calcium?

Here's our solution: We need the periodic table to determine the molar mass of calcium and its valence charge.

The molar mass of calcium is 40.78 g and its valence charge is +2. Its equivalent weight is solved as follows:

Here's another example we can look at. We begin with the following prompt: What is the equivalent weight of nitrogen?

Here's our solution: Nitrogen is in family 5A (main-group-element 5). Every element in this family has a valence charge of 5 - 8 = -3. We only need the magnitude of this charge so the number of equivalents is simply 3. The molar mass of nitrogen is 14.01 g. The equivalent weight is calculated as follows:

Acids and Bases

The n number of equivalents in an acid or base is the number of hydrogen ions (H+1 ) in an acid and the number of hydroxide ions (OH-1 ) in a base.

Here's an example we can use. Let's begin again with a prompt: What is the equivalent weight of calcium hydroxide?

Here's our solution: Calcium hydroxide has the formula Ca(OH)2 , so the n number of equivalents is 2 due to the 2 hydroxide ions in the formula. The molar mass is 74.1g.

The equivalent weight can now be determined. It can be solved by:

Here's another example. Our prompt is: What is the equivalent weight of phosphoric acid?

Here's our solution: The formula for phosphoric acid is H3 PO4 . The number of equivalents of hydrogen ions is 3. The molar mass is 98g. Its molar mass is solved by the equation:

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