Normality Formula

Normality like Molarity is a way of expressing the concentration of solute in a solution. If you can recollect, Molarity was defined as the number of Moles (n) of solute present in 1lt. of the solution. Now to define ‘Normality’ or ‘equivalent concentration’, instead of using the number of Moles (n), we will use the number of equivalents (eq) of solutes.

So, Normality (N) = \[\frac{\text{Number of equivalents}}{\text{Volume in litres}}\] 

Now, calculating the number of equivalents is similar to calculating number of Moles

Number Of Moles(n)= \[\frac{\text{Given mass}}{\text{molar mass}}\]

Number Of Equivalents(Eq)= \[\frac{\text{Given mass}}{\text{equivalent Mass}}\]

The next problem is how do we find the Equivalent Mass or Equivalent Weight (E.W) or gram equivalent.

Equivalent Weight is defined as Molar Mass divided by equivalence factor (f).

\[E.W = \frac{\text{Molar Mass}}{\text{Valency Factor(f)}}\]

This equivalence factor will differ for different species and also the type of reaction under consideration.

But let us understand this with a very easy example:

Case I:

The equivalent weight of NaCl (ionic salt)

The valency factor (f) of an ionic salt is always equal to the total charge on anion ( in this case) or cation ( in this case)

So if you can observe, the valency factor (f) for NaCl =1.

Hence its Equivalent Weight = \[E.W = \frac{\ext{Molar Mass}}{\text{Valency Factor(f)}}\]

\[\frac{56.5}{1}=56.5\]

Similarly, for MgCl2, valency factor (f) = 2

And \[E.W=\frac{95}{2}=47.5\]

Case II:

For acids or bases, the valency factor (f) is equal to the number of Moles of

H+orOH-

ions 1 Mole of acid or base can furnish in an aqueous medium.

e.g. 1 Mole of H2SO4 will furnish 2 Moles of ions, so its (f) = 2, E.W = 98/2 = 49 gm/eq

1 Mole of NaOH will furnish 1 Mole of ions, so its (f) = 1, E.W = 40/1 = 40 gm/eq

1 Mole of H3PO3 will produce only 2 Moles of ions (check the structure of acid), so its (f) = 2,

E.W = 82/2 = 41 

So, what we can gather from here is that we need to know the acidity of the base and the basicity of acid to determine the valency factor (f)

Case III:

For Redox Reaction, the (f) factor for any species is the total change in the oxidation state it has undergone in that reaction.

e.g. KMnO4 as an oxidising agent in acidic media

MnO4−+8H++5e−→Mn2++4H2O

KMnO4 gained 5 electrons from the reductant. 

Therefore (f) = 5

Hence E.W = 158.04/5 = 31.61 grams/equivalent.

I know there is a lot to remember in this and it looks more tricky than calculating Molarity, but with a few examples and little practice, you will get a hang of it!

Example: What is the Normality of the following of 1lt. an aqueous solution containing 80 gm of NaOH dissolved in it.

Solution: 

Firstly, In order to determine the Normality (N), we need to find the number of equivalents.

Now we know that the (f) for NaOH = 1

E.W = 40/1 = 40gm

Number of Equivalents(Eq)= \[\frac{\text{Given mass}}{\text{equivalent Mass}}\]

 = 80/40 = 2 Eq

Hence, N =

\[(N)=\frac{\text{Number of equivalents}}{\text{Volume in litres}}\] = 2/1 = 2 Eq/lt.

1. What is the Normality of 0.1381 M NaOH?

Options:

(a) 0.5231 N

(b) 0.6123 N

(c) 0.7775 N

(d) 0.1381

Ans: (d)

Solution: 

0.1381 mol/L x (1 eq/1mol) = 0.1381 eq/L = 0.1381 N 

Since the Equivalent Weight of NaOH is equal to Molar Mass hence the Molarity is equal to Normality in this case.

More About Normality Formula

Normality Formula along with its definition is available for a free download at Vedantu.

The amount of a substance in a specific Volume of solution is called Molarity and it is represented by M. The Moles of the solute in solutions per liter are called Molarity, which is also sometimes referred to as the molar concentration of a solution, that is to say, it is a way of expressing the concentration of a solute in a solution.

Now coming to the Normality, it is also, just like the Molarity, a method for expressing the concentration of solute in a solution. And as we have just seen that Molarity is defined by the number of Moles present in the one-liter solute, the Normality is defined by the number of equivalents of solutes. Normality can also be referred to as an Equivalent concentration.

Usage of Normality Formula or the Equivalent Concentration

There are many usages of the Normality formula or the equivalent concentration, some of which are mentioned below.

• The Normality formula is used to express the hydronium ions concentration, or hydroxide ions concentration in a solution, in acid-base chemistry.

• The Normality formula describes the capacity of accepting or donating the number of electrons that an oxidizing or the reducing agent can manage, in a redox reaction.

• In the precipitation reactions, which is to say, the process of morphing the substance which is dissolved into an insoluble solid from the solution which is supersaturated. In such reactions, Normality is used to describe the number of electrons that can be accepted or donated by the reducing or an oxidizing agent.