Defining Equivalent Weight
Equivalent weight, also known as gram equivalent, is the mass of a substance that will combine with or displace a fixed quantity of another substance. Historically, it was defined based on 1.008 grams of hydrogen, 8 grams of oxygen, or 35.5 grams of chlorine. In modern chemistry, it's the mass of a substance that reacts with or provides one mole of hydrogen ions (H+), hydroxide ions (OH-), or electrons in a redox reaction, typically expressed in grams per equivalent (g/eq).
Calculation Methods for Various Reactions
The calculation of equivalent weight depends on the type of chemical reaction. For an acid, it's the molar mass divided by the number of replaceable hydrogen atoms (basicity). For a base, it's the molar mass divided by the number of replaceable hydroxide ions (acidity). In redox reactions, the equivalent weight is the molar mass divided by the number of electrons gained or lost per mole of the substance, which is often referred to as the 'n-factor' or 'valence factor'.
Practical Example: Sulfuric Acid
Consider sulfuric acid (H₂SO₄). Its molar mass is approximately 98.08 g/mol. In a neutralization reaction where both hydrogen atoms are replaced, its equivalent weight is 98.08 g/mol / 2 = 49.04 g/eq. This means 49.04 grams of H₂SO₄ can react with one mole of hydroxide ions. If it only reacted to replace one hydrogen, its equivalent weight would be 98.08 g/eq in that specific context.
Importance and Distinction from Molar Mass
Equivalent weight is important because it simplifies stoichiometric calculations, especially in acid-base titrations and redox reactions, by allowing direct comparisons of reactive capacities. Unlike molar mass, which is a fixed property of a compound, the equivalent weight of a substance can vary depending on the specific reaction it undergoes. It provides a way to express the 'reacting power' of a substance in a given chemical process.