Equivalent Weight of an Acid
In an acid base titration, the equivalence point is the volume of added base where the moles of –OH added (from the base) equal the moles of H+ initially present (from the acid). (i.e) moles of H+ initially present = moles –OH added (at equivalence point)
To approximate the equivalence point, an indicator with an endpoint close to the equivalence point is added to the analyte solution. A balanced equation can be written describing the chemical reaction occurring between the titrant (the base in this experiment) and analyte (the acid in this experiment) if the identity of both is known.
For example, the titration of hydrochloric acid with potassium hydroxide can be written:
HCl(aq) + KOH(aq) ⎯→ H2O + KCl(aq)
In the case if the identity of the acid is unknown, but the number of acidic hydrogens (H+) carried by the acid is known, a balanced equation can still be written.
For example, the titration of a triprotic acid (an acid with 3 H+) with sodium hydroxide can be written:
H3X(aq) + 3KOH(aq) ⎯→ 3H2O(l) + K3X(aq)
(X : the unknown anion of the acid)
The formula weight of this unknown acid can be calculated by using dimensional analysis. First, the base’s concentration is used to convert the base’s volume at the endpoint to moles. Then, multiplying by the mole ratio between acid and base
from the balanced chemical equation allows for the calculation of the moles of the acid. Now the mass of acid titrated must be divided by the moles of acid calculated giving a result with the units of g/mol.
Equivalent Weight of an Oxidizing Agent
The concept of equivalents and equivalent mass is not restricted to acid-base reactions alone. Unlike acid-base reactions in redox reactions, the electrons are the active units (the equivalents) and the equivalent weights are the masses of oxidizing or reducing agent that deliver or accept 1 mole of electrons. But in case of acid and base the hydrogen or hydroxide ions plays key role in determination of equivalent weight.
QUESTIONS
1 The equivalent weight is determined by dividing the atomic or molecular weight by the ……………….
2 The mass of a substance especially in grams is chemically equivalent to ………………. grams of oxygen
(a) 2 (b) 5 (c) 8 (d) 10
3 ………………. is the primary standard reagent commonly used to standardize NaOH3 Molarity
