Equivalent Weight

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What is Equivalent Weight?

Equivalent weight in chemistry is the most common term used in solution and some basic concept of chemistry in the physical chemistry part.  Here, we will have covered the important topics related to the equivalent weight of metal. What is the equivalent weight definition in chemistry? The amount of substances that completely react with each other in the reaction is called equivalent weight in chemistry. While answering the question what is equivalent weight? you should keep in mind that its definition depends on the two factors; the molar mass and valency factor of the compound.


Equivalent Weight

Equivalent weight = molecular weight / X

In the above formula X represents the valency factor.


For Acids

1. Sulphuric Acid (H2SO4)

For acids, X (valency factor) is the basicity

Basicity- basicity is the number of hydrogen ions or hydronium ions released by an acid.

In the case of sulphuric acid (H2SO4)

H2SO4 → 2H+ + SO4-2

The number of hydrogen ion released by sulphuric acid is 2. Therefore, its valency factor or X value will be two.

The molecular weight of sulphuric acid is 94.

As we know, Equivalent weight = molecular weight / X

The equivalent weight of sulphuric acid = 98 / 2 = 49


2. Hydrochloric Acid

In the case of hydrochloric acid (HCl)

HCl → H+ + Cl-

The number of hydrogen ion or hydronium ion released by hydrochloric acid is one.

So, the valency factor will be one.

The molecular weight of hydrochloric acid = 36.45

As we know, Equivalent weight = molecular weight / X

The equivalent weight of hydrochloric acid = 36.45 / 1 = 36.45


For Bases

1. Calcium Hydroxide Ca(OH)2

For the base, X (valency factor) is the acidity

Acidity- Acidity is the number of hydroxyl ions or hydroxide ions released by a base.

In the case of calcium hydroxide base Ca(OH)2

Ca(OH)2 →Ca+ + 2 OH-

The number of hydroxyl ions released by the calcium hydroxide base is 2. Therefore, its valency factor or X value will be two.

The molecular weight of the calcium hydroxide base is 74.

As we know, Equivalent weight = molecular weight / X

The equivalent weight of calcium hydroxide base= 74 / 2 = 37


2. Aluminium Hydroxide Al(OH)3

In the case of aluminium hydroxide base Ca(OH)2

Al(OH)3 → Al+3 + 3 OH-

The number of hydroxyl ions released by the aluminium hydroxide base is 3. Therefore, its valency factor or X value will be three.

The molecular weight of the aluminium hydroxide base is 78 g/mol.

As we know, Equivalent weight = molecular weight / X

The equivalent weight of calcium hydroxide base= 78 / 3 = 26.


Equivalent Weight of the Metal in Salt or Compounds 

1. Aluminium Chloride

Step by Step Calculation for Finding the Equivalent Weight of Aluminium Salts

For the metals, X (valency factor) is the total positive charge on the positive ion (cation).

In the case of aluminium chloride salt Al(Cl)3

AlCl3 → Al+3 + 3Cl-

The number of positive charge on aluminium cation is three. Therefore, its valency factor or X value will be three.

The molecular weight of the calcium hydroxide base is 133.34 g/mol.

As we know, Equivalent weight = molecular weight / X

The equivalent weight of aluminium chloride salt= 133.34 / 3 = 44.44.


2. Silver Carbonate (Ag2CO3)

Step by step calculation for finding the equivalent weight of silver salts

For the salts, X (valency factor) is the total positive charge on the positive ion (cation).

In the case of silver carbonate salt Ag2CO3

Ag2CO3 → 2Ag+ + CO3-

The number of the total positive charge of the silver cation is two. Therefore, its valency factor or X value will be two.

The molecular weight of the silver carbonate salt is 275.75 g/mol.

As we know, Equivalent weight = molecular weight / X

The equivalent weight of silver carbonate salt= 275.75 / 2 = 137.87.


Did You Know?

Equivalent weight is used in the calculation of normality. The normality of a solution is defined as the number of gram equivalents of the solute present per litre of the solution. It is represented by the symbol, N.

Normality = gram equivalent of the solute/volume of the solution in litres

The number of gram equivalents of the solute is calculated as follows:

No. of gram equivalents = mass of solute in grams / equivalent mass of the solute.


Summary

The equivalent masses of acids, bases, and salts are calculated as follows:

  • Equivalent mass of an acid = molecular mass of the acid/basicity.

  • Equivalent mass of a base = molecular mass of the base/acidity.

  • Equivalent mass of a salt = molecular mass of the salt/total positive valency of metal atoms.

FAQ (Frequently Asked Questions)

1. What is meant by equivalent weight?

The number of substances that completely react with each other in the reaction is called equivalent weight. In calculating the normality of a solution, the equivalent weight is required.

2. What are the factors on which equivalent weight depend?

The factors on which equivalent weight depend are given below

  • Molecular weight 

  • Valency factor