Equivalent Weight
Definition and meaning of Equivalent Weight in chemistry.
Equivalent weight is the mass of a chemical that provides one mole of reactive pieces. In an acid, this reactive piece is a single hydrogen ion, while in an electron transfer reaction, the piece is a single electron. This weight helps chemists figure out exactly how much chemical they need.
In more detail
You calculate this value by dividing the standard molar mass by a special number. This special number is called the equivalence factor or the n-factor. The n-factor simply counts how many reactive pieces each single molecule can give.
A simple acid that gives one hydrogen ion has an n-factor of one. A stronger acid that gives two hydrogen ions has an n-factor of two. A chemical that trades three electrons in a reaction has an n-factor of three.
A single chemical can actually have different equivalent weights in different experiments. It completely depends on how the chemical acts in that specific chemical reaction. Sometimes a molecule might only give up half of its available hydrogen ions.
Chemists use this weight to create a special concentration scale called normality. Normality is very helpful when doing careful liquid mixing experiments called analytical titrations. It ensures that the mixing liquids always match up perfectly piece for piece. This makes the math much easier than using normal moles and molarity.
Key facts
| Field | General Chemistry |
|---|---|
| Standard Formula | Equivalent weight equals molar mass divided by n-factor |
| Standard Unit | Grams per equivalent (g/eq) |
| Acid n-factor | Number of hydrogen ions donated per molecule |
| Base n-factor | Number of hydroxide ions donated per molecule |
| Redox n-factor | Number of electrons transferred in the reaction |
Sulfuric acid has the chemical formula H2SO4 and a molar mass of 98.08 grams. During a full neutralization, each molecule gives away both of its hydrogen ions. This gives the acid an n-factor of exactly two for this specific reaction. We divide the starting molar mass of 98.08 by the n-factor of two. This means the equivalent weight of sulfuric acid is 49.04 grams per equivalent. If we only need one mole of hydrogen ions, we only need 49.04 grams.
Frequently asked questions
How does equivalent weight relate to normality?
Normality is a concentration scale based on equivalents instead of moles. Equivalent weight is just the physical mass of one single equivalent.
Why can one compound have more than one equivalent weight?
A chemical might act differently depending on the exact reaction. Phosphoric acid can give one, two, or three protons depending on the situation.
Why do chemists use equivalent weight instead of molar mass?
It lets chemists compare different chemicals directly based on their reactive power. It makes balancing titration math much faster and simpler.