Definitions of Equivalent Weight ( Acid-Base Context )

The weight of a compound that contains ONE EQUIVALENT of a proton (for acid) or ONE EQUIVALENT of an hydroxide (for base).

Examples:

(1) H₂SO₄ + 2OH^-= 2H₂O + SO₄^2-

Molecular weight of sulfuric acid (H₂SO₄) = 98.07 g/mol

Theoretical Equivalent weight of sulfuric acid (H₂SO₄) = 49.03 g/equivalent of H⁺

Reasoning:

98.07 g/mol H₂SO₄ * (1 mole H₂SO₄ / 2 equivalents of H⁺) = 49.03 g/equivalent of H⁺

In this example, the magnitude of the equivalent weight of sulfuric acid is HALF of that of the molecular weight. This is beacuse according to the balanced chemical reaction, one mole of sulfuric acid reacts with TWO equivalents of hydroxide (i.e. same as saying that there are TWO equivalents of protons per mole of sulfuric acid).

(2) NH₄OH + H⁺ = H₂O + NH₄⁺

Molecular weight of ammonium hydroxide (NH₄OH) = 35.00 g/mol

Theoretical Equivalent weight of ammonium hydroxide (NH₄OH) = 35.00 g/equivalent of OH^-

Reasoning:

35.00g/mol of NH₄OH * (1 mole NH₄OH / 1 equivalent of OH^-) = 35.00 g/equivalent of OH^-

In this example, the magnitude of the equivalent weight of ammonium hydroxide is the same as the molecular weight.This is beacuse according to the balanced chemical reaction, one mole of ammonium hydroxide reacts with ONE equivalent of H⁺.

(3) H₂SO₄ + OH^-= H₂O + HSO₄^-

Molecular weight of sulfuric acid (H₂SO₄) = 98.07 g/mol

Theoretical Equivalent weight of sulfuric acid (H₂SO₄) = 98.07g/equivalent of H⁺

Reasoning:

98.07 g/mol H₂SO₄ * (1 mole H₂SO₄ / 1 equivalent of H⁺) = 98.07g/equivalent of H⁺

In this example, the magnitude of the equivalent weight of sulfuric acid is the same as the molecular weight. This is beacuse according to the balanced chemical reaction, one mole of sulfuric acid reacts with ONE equivalent of hydroxide.

Note: In a laboratory setting, one can obtain the EXPERIMENTAL EQUIVALENT WEIGHT simply by analyzing a known amount of sample by using volumetric analysis. In other words, by knowing the weight of the sample used in the analysis and the number of equivalents of the titrant used in the titration, one can then calculate the experimental equivalent weight based on the data.

Once the experimental equivalent weight is known, one can then compare the theoretical equivalent weight (i.e. one that based only one the balanced chemical reaction) with the experimental one.