# Stiochiometry of Redox Reaction

In general, in a chemical reaction described by the balanced chemical equation:

AA + bB + â†’ cC + dD

A, B .... are the reactants and C, D ....... are the products. The coefficients a,b,c and d are known as stoichiometric coefficients. These indicate the number of moles of the reactants used and the number of moles of the products obtained in the reaction.

In the preceding sections we have learnt to balance the chemical equations for the redox equations and obtain the stoichiometric coefficients of all the reactants and products. These coefficients must be kept in mind in all the stoichiometric calculations. They are quite important in volumetric titrations based on redox reactions. The acidic permanganate is used to estimate ferrous ion. If we use KMnO

_{4}and FeSO

_{4}. 7H

_{2}O as the reactants then we know that for one mole of potassium permanganate we use 5 moles of ferrous sulphate according to the balanced stoichiometric equations. This fact was taken into account earlier by introducing the concept of equivalent weight and normality. However, in view of the more acceptable mole concept, calculations are done on the basis of mole concept and molarity of the solution.

Volumetric titrations based on redox reactions are carried out routinely in chemical laboratories. If we know the molarities of the reductant and oxidant and their volumes used in titration and if n

_{1}and n

_{2}are their stoichiometric coefficients then it is possible to use the equation

= for volumetric estimation.

for example, in the titration of FeSO

_{4}with KMnO

_{4}the balanced chemical equation is:

10FeSO

_{4}+2KMnO

_{4}+ 8H

_{2}SO

_{4}â†’ 5Fe

_{2}(SO

_{4})

_{3}+ K

_{2}SO

_{4}+ 2Mn

_{2}SO

_{4}+ 8H

_{2}O

If FeSO

_{4}is reagent '1' and KMnO

_{4}is reagent 2 then equation becomes

= or = and can be used to calculate the unknown quantity if the other three quantities are known.