The most commonly used phrase in chemistry is “equivalent weight,” which is also one of the fundamental concepts in physical chemistry. Equivalent weight commonly referred to as a gram equivalent is the mass of one equivalent or the mass of a particular material that will combine with or replace a specific amount of another substance. To say it another way, the mass of a substance that can displace 1.008 grams of hydrogen or 8.0 grams of oxygen or 35.5 grams of chlorine is its gram equivalent or equivalent weight. Let us study the Equivalent Weight in detail in this article. 

What is Equivalent Weight?

Equivalent weight is defined as the ratio of the molecular weight of the solute to the valency of the solute. The equivalent weight of the substance varies according to the reaction it undergoes. The normality of the solution is calculated using the equivalent weight of the substance. The general formula to calculate the equivalent weight is,

Equivalent Weight Formula

Equivalent Weight = Molecular Weight / (n-factor)Valency

How to calculate the Equivalent Weight?

The Equivalent Weight of a compound is the result of dividing the molecular weight by the charge number of that compound.

E = Molecular Weight / n-factor (charge number)

where, 
E is Equivalent Weight
Charge number indicates how many protons or hydroxide-equivalents the compound has

The equivalent weight of the compound can be defined as the weight (or mass) of a substance that will include a single reactive proton (or hydrogen ion, H⁺) or a single reactive hydroxide ion (OH^–). The idea of equivalent weight is necessary because some compounds are doubly reactive for every mole present because they have the capacity to supply or receive more than one proton.

Hydrogen has an atomic mass of one (1.008 to be precise). Hydrogen’s valency is always 1. As a result, hydrogen has an equal weight of 1. In the majority of reactions, oxygen has a valency of 2.

Moles

A compound is described as having 6.02 × 10²³ distinct particles (atoms or molecules) per unit mole. In fact, there are exactly this many atoms in 12 grams of carbon. The mass of one mole of each element, or its molecular weight (MW), is given in the corresponding box for that element on the periodic table.

Equivalent Weight of Acids and Bases

The equivalent mass of an acid or base in an acid-base reaction is always equal to the mass that contributes to or interacts with one mole of the hydrogen ion (H+). In a similar manner, the mass that supplies or reacts with one gram mole of electrons (e-) produced in the redox reaction is the substance’s equivalent weight.

Knowing an acid’s molecular weight and the charge present in it helps to calculate its equivalent weight.

For example, take sulfuric acid,

H₂SO₄ + 2OH⁻ → 2H₂O + SO₄²⁻

Using a periodic table to get each element’s MW and adding it we get 2(1) + (32) + 4(16) = 98.0, and we can determine the acid’s Molecular Weight.

Because the sulfate ion is left with a charge of 2, we get to know that this acid can give two protons. 

Hence, the equivalent weight = 98.0/2 = 49.0

Logic is the same for a base. A proton can be taken up by ammonium hydroxide in solution to form an ammonium ion:

NH₄OH + H⁺ = H₂O + NH₄⁺

Ammonium hydroxide’s Molecular Weight is calculated as (14) + (4)(1) + (16) + 1 = 35.0

The equivalent Weight for this molecule is 35.0/1 = 35.0 since just one proton is consumed.

How to Calculate Gram-Equivalent Weight?

The number of grams of a substance divided by its equivalent weight is known as a gram equivalent. It can also be written as n moles times the number of charge elements present.

Gram equivalent weight is the equivalent weight given in mass units. The calculated equivalent weight and the gram equivalent weight are equal numerically.

E_q = MW / n

where,
E_q is Equivalent weight
MW is Molecular Weight in grams/mole
n is number of equivalents charge

Also, Read

• Normality
• Molarity
• Difference Between Molarity and Molality
• Equivalent Conductance Formula
• Weight Formula

Solved Problems on Equivalent Weight

Question 1: Calculate the Equivalent weight of H₂SO₄.

Answer:

Two H+ ions are present for every mole of sulfuric acid, or n = 2. Find the total atomic masses of the S, O, and H in your formula by referencing a periodic table:

S = 32.07

O = 16.00

H = 1.01

Add 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.

H₂SO₄ has a gram equivalent weight of 49.04 g/eq.

Question 2: Calculate the Equivalent weight of NaOH.

Answer:

Since there is just one OH-, there are only one equivalent. Find the total atomic masses of the Na, O, and H in your formula by referencing to periodic table:

Na = 22.99

O = 16.00

H = 1.01

Add the molecular weight of NaOH: 

22.99 + 16.00 + 1.01 = 40.00 g/mole

Eq = 40.00 / 1 = 40.00 g/eq

NaOH has a gram equivalent weight of 40.00 g/eq

Question 3: Calculate the Equivalent weight of HCL.

Answer:

Since there is just one H⁺, there are only one equivalent. Find the total atomic masses of Cl and H in your formula by referencing to periodic table:

Cl = 35.45

H = 1.01

Add the molecular weight of HCL:

35.45 + 1.01 = 36.46 g/mole

Eq = 36.46 / 1 = 36.46 g/eq

HCL has a gram equivalent weight of 36.46 g/eq

Question 4: Calculate the Equivalent weight of Ca(OH)₂ →Ca⁺ + 2 OH^–

Answer:

The calcium hydroxide base releases two hydroxyl ions into the environment. Its valency factor, or X value, will therefore be 2.

The calcium hydroxide base has a molecular weight of 74 g/mole.

Equivalent weight = molecular weight / valency

Equivalent weight of calcium hydroxide base= 74 / 2 = 37 g/eq

Therefore, calcium hydroxide has a equivalent weight of 37 g/eq

Question 5: Calculate the Equivalent weight of Al(OH)₃ → Al⁺3 + 3 OH^–

Answer:

The aluminum hydroxide base releases three hydroxyl ions. Its valency factor, or X value, will therefore 3.

The aluminum hydroxide base has a molecular weight of 78 g/mol.

Equivalent weight = molecular weight / valency

Equivalent weight of aluminum hydroxide base= 78 / 3 = 26 g/eq.

Therefore, aluminum hydroxide has a equivalent weight of 26 g/eq

Equivalent Weight – FAQs

What is equivalent weight?

Equivalent weight is the mass of a substance that combines with or is equivalent to eight grams of oxygen or one gram of hydrogen in a chemical reaction.

How is equivalent weight calculated?

It is calculated by dividing the molecular weight by the valence (number of electrons transferred per molecule in the reaction).

What is the relationship between equivalent weight and molarity?

Equivalent weight helps in converting molarity to normality by accounting for the number of ions participating in a reaction.

When is equivalent weight used in calculations?

It is used in titration calculations and in determining the concentrations of solutions in acid-base reactions.

Why is equivalent weight important in redox reactions?

It allows for the calculation of the exact amount of a substance needed to react with a given amount of another substance in redox reactions.

Can equivalent weight vary with the type of reaction?

Yes, the equivalent weight of a substance can vary depending on the reaction it participates in due to different valences.

How does equivalent weight relate to normality?

Normality is the number of equivalents per liter of solution, and it directly uses the concept of equivalent weight for its calculation.

What is the difference between equivalent weight and molar mass?

Molar mass is the mass of one mole of a substance, while equivalent weight considers the mass that reacts with a fixed amount of another substance based on valence.

How do you calculate the equivalent weight of acid and base?

Divide the molar mass of the base by the quantity of hydroxyl groups to determine the equivalent mass of the base. Divide the molar mass of an acid by the number of protons to determine its equivalent weight.

What is the equivalent concentration, and how to calculate it?

EW stands for equivalent weight in grams per equivalent. It is computed by dividing the solute’s molecular weight by its equivalents per mole. The amount of H+ ions that an acid contributes to a mole of acid determines how many equivalents are there for acids.

What does “equivalent concentration” mean?

The molar concentration of a solution (ci), is divided by an equivalency factor (f), to determine its equivalent concentration, or normalcy. Normality = cif

What are the factors on which the equivalent weight of any compound depends?

The equivalent weight of any compound depends upon the following two factors,

• Molecular weight 
• Valency factor