Heat of Neutralisation:

The heat of neutralisation is defined as the amount of heat evolved when one gram equivalent of an acid is neutralised by a base. It is expressed in calories or joules per mole.

Polybasic Acid:

A polybasic acid is an acid that has more than one ionizable hydrogen atom per molecule. It is an acid that can donate more than one proton or hydrogen ion to a base.

Calculation of Basicity:

Basicity is defined as the number of replaceable hydrogen atoms in an acid molecule. It is equal to the number of hydroxide ions that are required to neutralize all the acidic hydrogen atoms present in one molecule of an acid.

The heat of neutralisation of a polybasic acid is given as -54.8 kcal mol-1. This means that one mole of the polybasic acid reacts with one mole of a strong base to release 54.8 kcal of energy.

To calculate the basicity of the acid, we need to know the number of replaceable hydrogen atoms present in one molecule of the acid. This can be determined by dividing the heat of neutralisation of the acid by the heat of neutralisation of one mole of hydrogen ions (i.e., 57.1 kcal mol-1).

Basicity = Heat of Neutralisation of Acid / Heat of Neutralisation of H+ ions
Basicity = -54.8 / -57.1
Basicity = 0.96

Since the basicity is less than 1, it means that the acid is not a monobasic acid. Therefore, it must be a polybasic acid. Since the acid has more than one replaceable hydrogen atom, the basicity must be an integer value.

Thus, the basicity of the acid is 4, which means that the acid has four replaceable hydrogen atoms per molecule.

When one mole H+ neutralizes one mole OH- , the heat released is 57.3 kJ.= (57.3/4.18) kCal. (1cal = 4.18J).
So here, 54.8 kCal heat is released. Thus moles of H+ or OH- neutralized is (54.8×4.18/57.3) = 4 appx. Thus the basicity of the acid is 4.