Normality

What is Normality?

We define normality as the number of gram equivalent of solute that is present in a one-litre solution. So, the unit of normality is gram/litre.

We denote normality with a letter ‘N’.

We can write the Normality Formula as:       

  N  = \[\frac{\text{No. of gram equivalent of the solute}}{\text{volume of the solution in Liters}}\]

Now, you might be wondering what gram equivalent is and how it is related to the number of moles.

Let’s understand what is equivalent and its significance as well.

Let’s say we have an equation:

       NaCl              +     H2SO4          →     Na2SO4                +      HCl

(Sodium Chloride) (Sulphuric Acid)   (Sodium Sulfate)    (Hydrochloric Acid)


Equivalence

You can see that this equation is not balanced. Now, let’s balance this equation:

                                 2NaCl  + H2SO4 → Na2SO4 + 2HCl

No of moles (n):   (2 moles)   (1 mole)     (1 mole)  (2 moles)

So, 2 moles of NaCl reacts with 1 mole of H2SO4 to give 1 mole of Na2SO4 and 2 moles of HCl.

This means, without balancing the equation, we can’t determine the quantity of reactant (or moles) that undergoes a reaction to form a product.


Number of Gram Equivalent

So, let’s get forward with understanding the concept of gram equivalence.

We know that no of moles =\[\frac{Mass}{\text{Molecular weight}}\]

Number of gram equivalent = \[\frac{Mass}{\text{Equivalent weight}}\], and equivalent weight = \[\frac{\text{Molecular weight}}{X}\]

(X = valance factor, where valence factor for acids and bases is the number of H+ and OH- ions they release in the solution, respectively).

We’ll understand these two formulas with an example.

Let’s find out Gram Equivalent

For example, Find the number of gram equivalents present in 0.5 g of HCl.

HCl releases one H+ ion in the solution, so its valence factor = 1.

The molecular weight of HCl = 36.46 g.

So, equivalent weight =  \[\frac{\text{Molecular weight}}{X}\] = 36.46/1 = 36.46 g, and

Number of gram equivalent = \[\frac{Mass}{\text{Equivalent weight}}\] = 0.5/36.46 = 0.0137

So, we get the number of gram equivalent  = 0.0137

Let us take another example of 1.06 g of Na2CO3 to understand this concept clearly

We are given the mass of Na2CO3 = 1.06 g.

Firstly, Find the equivalent weight of Na2CO3

Since Na2CO3 is a salt, so the number of positive charges on the cation gives X = 2

Molecular weight = 106 g

So, Equivalent Weight = \[\frac{\text{Molecular weight}}{X}\] = 106/2 = 53 g, and number of gram equivalent is:

= \[\frac{Mass}{\text{Equivalent weight}}\] = 1.06/53 = 0.02

In a chemical equation, the number of gram equivalent of both reactions always remains the same.


Normality Chemistry

There are three types of Normality

  1. Seminormal - The solutions whose normality is ½ or N/2.

  2. Binormal - The solutions having normality as 2 or 2 N.

  3. Decinormal - Normality is 1/10 or N/10.

  4. CentiNormal - Normality is 1/100 or N/100.


Normality Calculation Formula

Let’s take an example of how to calculate normality:

If 13 g of N2O4 is present in 500 ml of solution. Find normality.

We are given with mass of N2O4 = 0.65 g, and volume = 500 ml = 0.5 l.

We know that normality, N = no of gram equivalent/volume of solution in litres

Let’s find out equivalent weight to find out the number of gram equivalent for N2O4

Molecular weight of N2O4 = (2 x 14) + (4 x 16) = 28 + 64 = 92 g.

Since the number of negative charges on oxygen = 4. So, X = 4

So, equivalent weight = \[\frac{\text{Molecular weight}}{X}\]= 92/4 = 13 g, and

Number of gram equivalent = \[\frac{Mass}{\text{Equivalent weight}}\]= 13/13 = 1

Now, let’s calculate the normality, by the formula given below:

N =  No of gram equivalent / Volume of solution in liters = 1/0.5             

    N = 2 gram / liter.

Here, the Normality is N = 2, which means the solution of N2O4 is BiNormal.


Normality Equation

For deriving the normality equation, let’s understand normality in mixtures

Let’s consider two ideal solutions having their normalities as Na and Nb, and the volume as Va and Vb respectively as shown below:

Where, Na = The normality of the acidic solution,

             Va = Volume of the acidic solution, 

             Nb = Normality of the basic solution, and

             Vb = Volume of the basic solution.

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On combining these two solutions, we get a mixture whose volume is Va + Vb and the normality as N.

So, we got the Normality Formula for the mixture as:   

  N = (Na Va + Nb Vb)/ (Va + Vb)

Let’s consider three Cases:

Case 1: The concentration of the acidic solution > concentration of the basic solution (the release of H+ ions > OH- ions)

So,  Na Va >  Nb Vb

Case 2: The concentration of the basic solution < acidic solution, then

Na Va <  Nb Vb (release of OH- ions > H+ ions)

Case 3: When concentration is equal, then

 Na Va = Nb Vb is the normality equation.

In this case, there is no release of both OH- ions and H+ ions.

This means the number of gram equivalent of H+ ions = number of gram equivalent of OH- ions. Such a type of solution is neutral and this process is called the neutralization.

FAQ (Frequently Asked Questions)

Q1: What is the simple definition of the Gram Equivalent?

Ans: The gram equivalent is the mass or the amount of substance (ion or molecule) that will combine with or displace a fixed quantity of another substance.

Q2: Write an example of Normality.

Ans: We know that the normality of a solution is the gram equivalent of a solute in per litre solution.

Let’s say a molecule, HNO3 is having a normality of 0.5. So, we can express its concentration as 0.5 N HNO3.

Q3: What is the Formula for Molarity?

Ans: The Formula for Molarity is:

            Molarity (or M) = The number of moles of solute / Volume of solution in L.

Q4: Derive the relationship between Normality and Molarity.

Ans: We already know that:

Normality = No of gram equivalent in a solute / Volume of solution (in L), and 

Molarity (or M) = The number of moles of solute / Volume of solution in L

Now, N/M = No of gram equivalent / Number of moles of solute…(1)

Since number of Gram equivalent = Mass / Equivalent mass, and

Number of Moles = Mass / Molecular mass

From (1), N/  M= Molecular mass / Equivalent mass…(2), and

Equivalent mass = Molecular mass / X

From (2), we get,

                N / M = X

This means

N = M * X

Normality = Molarity * Valency factor