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