Normality and Gram Equivalent Weight

Normality

Normality is defined as the number of gram equivalents of a solute contained in 1 litre of a solution. Thus a solution which contains one gram equivalent of the solute per litre of the solution is called normal solution. It is represented as 1 N. 

Gram equivalent is the weight of substance in grams numerically equivalent to its equivalent weight.

Equivalent weights of Acids and Bases:

The equivalent weight of an acid is that weight of it which contains 1.008 gm of replaceable hydrogen. 

The equivalent weight of a base is that weight of it contains one replaceable hydroxyl group. 

For example, HCl and NaOH have only replaceable H⁺ and OH-hence the equivalent weights  of them are equal to their molecular weights. 

Equivalent weight of HCl

                        Molecular weight of HCl  =  H(1 X 1) + Cl(35.5 X 1)

                                                                  = 1+35.5 

                                                                  = 36.5

            Number of replaceable ions (H⁺)  = 1

                           Gram equivalent weight = molecular weight/number of replaceable ions

                           Gram equivalent weight = 36.5 gm

Equivalent weight of NaOH

                      Molecular weight of NaOH = Na (23 X 1) + O (16 X 1) + H (1 X 1)

                                                                   = 23 + 16 + 1

                                                                   = 40 gm    

            Number of replaceable ions (OH-) = 1

                             Gram equivalent weight = molecular weight/number of replaceable ions

                            Gram equivalent weight  = 40 gm

                                       

Sulphuric acid (H₂SO₄) and Oxalic acid (HOOC-COOH) each have two replaceable hydrogen, hence their equivalent weights are equal to half of their respective molecular weights.

Equivalent weights of some common Acids:

Acidity: No. of replaceable OH- ions.

 Basicity : No. of replaceable H⁺ ions.

Acid	Molecular weight	Basicity	Equivalent weight
Hydro Chloric Acid, HCl	36.46	1	36.46
Nitric Acid, HNO₃	63.01	1	63.01
Sulphuric Acid, H₂SO₄	98.07	2	49.03
Acetic Acid, CH₃COOH	60.05	1	60.05
Boric Acid, H₃BO₃	61.83	3	20.61
Oxalic Acid (dihydrate), (COOH)₃,2H₂O	126.07	2	63.03

Equivalent weights of some common Bases:

Base	Molecular weight	Acidity	Equivalent weight
Sodium Hydroxide, NaOH	40.00	1	40.00
Potassium Hydroxide, KOH	56.11	1	56.11
Sodium Carbonate, Na₂CO₃	105.99	2	53.00
Sodium bicarbonate, NaHCO₃	84.01	1	84.01
Potassium Carbonate, K₂CO₃	138.21	2	69.10
Potassium bicarbonate, KHCO₃	100.12	1	100.12

Specific gravities, percentage and approximate Normalities of some common Acids:

Acid	Specific gravity	Percentage	Approximate normality
Concentrated sulphuric acid	1.84	Not less then 97.0 w/w H₂SO₄	36N
Concentrated hydrochloric acid	1.18	35.0 – 38.0 w/w HCL	11.5N
Concentrated nitric acid	1.42	69.0 – 71.0 w/w HNO₃	16N
Acetic acid (glacial)	1.05	Not less then 99.0 w/w C₂H₄O₂	17.5N

How to Prepare 0.5 N and 1 N HCl

Concentrated HCl

      Concentrated HCl has strength of approximately 31-36%,in case of 36% means 360 ml HCl is present  per 1000 ml of solution for 32 % HCl, 320 ml of HCl is present per 1000 ml of solution. 

      For preparing dilute normal solutions from concentrated HCl we need to calculate its volume according to its percent strength required to mix with water. To calculate required volume of concentrated HCl to prepare different normal solutions  follow below mentioned steps.  normally HCl found in chemistry labs is of 36% so let us take example of 36% HCl for calculation.

Step 1

Calculate weight of HCl in 360 ml of HCl by multiplying it with specific gravity of 36% HCl i.e. 1.1789

360 x 1.178=424.08 gm

 means 360 ml of HCl = 424.08 gm

Step 2

Calculate no. of moles in 424.08 gm of HCl by using formula

                                   No. of moles =given weight/molecular mass of HCl

                                                     N=424.08/36.5 =11.6 moles

as HCl has only one replaceable Hydrogen ion its gram equivalent weight and molcular weight is same and hence Normality and Molarity is same as well. Means no. of moles is equal to gram equivalents which ultimately is its Normailty.

Hence, Normality of 36% HCl = No.of moles = 11.6 N

Step 3

Prepare 1N HCl by using Normality equation- 

let us calculate volume of concentrated HCl required to prepare 100 ml 1 N HCl.

(Conc. HCl)N1 V1= N2V2 (1N HCl)

11.6N x V1= 1N x 100ml

V1 =1 x 100/11.6

V1  = 8.6 ml

So to prepare 100 ml 1 N HCl from concentrated HCl  8.6 ml of concentrated Acid is diluted up to 100 ml with water.

Different normal solutions can be prepared  by using the above equation by changing the value of required volume and normality of the required solution .e.g.

To prepare 500 ml of 0.5 N HCl

(Conc. HCl)N1 V1= N2V2 (1N HCl)

11.6 N x V1= 0.5 N  x 500 ml

V1 =0.5 x 500/11.6

V1  = 21.5 ml

So to prepare 500 ml of 0.5 N HCl, 21.5 ml of concentrated Acid is diluted up to 500 ml.

It is not necessary to calculate every time no. of moles to calculate volume if u remember Normality of concentrated HCl i.e. 11.6 N, then volume can be easily calculated by direct from normality equation for different volume and normality.