Class 11- Redox reaction /What is equivalent weight/ How to find Equivalent weight / NEET JEE

What Is Equivalent Weight?

   • Q.How to find equivalent weight and v...  

The equivalent weight can be thought of as the weight (or mass, to be precise) of a substance that will contain a single reactive proton (or hydrogen ion, H+) or a single reactive hydroxide ion (−OH−). The former case applies to acids, which are proton donors, while the second applies to bases, which are proton acceptors.

The reason the concept of equivalent weight is needed is that some compounds can donate or accept more than one proton, meaning that for every mole present, the substance is in effect doubly reactive.

The general number of equivalents formula is

E = MW/valency factor

Where MW is the molecular weight of the compound and charge number is the number of proton- or hydroxide-equivalents the compound contains. Examples with different acids and bases help illustrate how this works in practice.
Example 1: H2SO4

For every mole of sulfuric acid, there are two H+ ions, n = 2. Look at a periodic table and find the sum of the atomic masses of the S, O and H in your formula:

S = 32.07; O = 16.00; H = 1.01. Sum the molecular weight of H2SO4: 32.07 + 4(16.00) + 2(1.01) = 98.08 g/mol

Eq = 98.08 / 2 = 49.04 g/eq.

The gram equivalent weight of H2SO4 is 49.04 g/eq. Only half as much sulfuric acid than, for example, HCl, is needed to react something with an acid.

Example 2: NaOH

There is only 1 OH-, so the number of equivalents is 1. Look at a periodic table and find the sum of the atomic masses of the Na, O and H in your formula:

Na = 22.99; O = 16.00; H = 1.01. To sum, 22.99 + 16.00 + 1.01 = 40.00 g/mole

Eq = 40.00 / 1 = 40.00 g/eq

The gram equivalent weight of NaOH is 40.00 g/eq.

Equivalent mass of a substance = (Molecular mass of the substance) / Something

Now WHAT is this “something”?

This is where we have to break our heads on. Now,… Substance may be:

Acids
Bases
Salt
Oxidizing / Reducing agent
In all these cases, we have a different “something”.

Substance is an acid.
Equivalent mass = (Molecular mass) / Basicity

Basicity is the maximum no. of H+ ions liberated on dissociation.

Eg.- Sulfuric acid. Basicity is 2. So Equivalent mass = ( 98/2) = 49

Substance is a base.
Equivalent mass = (Molecular mass) / Acidicity

Acidicity is the maximum number of OH- ions liberated on dissociation.

Eg.- NaOH. Acidicity is 1. So Equivalent mass = 40/1 = 40

Substance is a salt.
Equivalent mass = (Molecular mass) / Valency

Let us take Na2CO3 which dissociates to give Na+ and (CO3)2-. When you balance the equation, we get that there are 2 cations and 2 anions. So valency is 2.

Substance is an oxidizing or reducing agent.
Equivalent mass = (Molecular mass) / Change in O.S. or oxidation state

Let us take KMnO4. We see that in any acidic medium, the O.S. of Mn changes from +7 to +2. So the O.S. changes by 5.

So, Equivalent mass of KMnO4 = 158 / 5 = 31.6