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Determining the Limiting and Excess Reagents in a Chemical Reaction

In many situations during the chemical reactions, one of the reactants is taken in excess. This is to ensure the completion of reaction. However, on completion of reaction some of the reactant, taken ‘in excess, is left over. For example. consider the combustion of hydrogen

2H2(g) + O2(g)            2H2O(g)

Suppose that 2 mole of H2 and 2 mole of O2 are available for reaction. It follows from the equation that only 1 mole or O2 is required for complete combustion of 2 mole of H2 1 mole of O 2 will, therefore, be left over on completion of the reaction. The amount of the product obtained is determined BH2  the amount of the reactant that is completely consumed in the reaction. This reactant is called the limiting reagent. Thus. limiting reagent may be defined as the reactant which is completely consumed during the reaction. The reactant which is not completely consumed is referred to as excess reactant.

In the above example, H2 is the limiting reagent and O2 is excess reactant. The amount of H2O formed will, therefore be determined by the amount of H2. Since 2 mole of H2 are taken, it will form 2 mole of H2O on combustion.



Percentage composition of a compound refers to the amount of various constituent elements present per 100 parts by mass of the substance. It Can be calculated from the formula

of the compound by using the following relation:

In case of minerals or ores the mass percentage of a particular constituent species in a formula can also be calculated using the similar relation: