In a reaction, quantitative relationships among the reactants & products is called ‘Stoichiometry’.

Stoicheion means element  & Metron means to measure.

The interpretation of the coefficients as the number of moles is the basis of stoichiometric calculations.

N2(g) + 3H2(g) —> 2NH3(g)

Mole ratio          1 mole     3 moles           2 moles

Weight ratio        28 g          6 g                  34 g

Volume ratio      1 vol           3 vol              2 vol

For stoichiometric calculation, the first step is to write the chemical equation for the reaction and balance the reaction. The balanced equation gives the stoichiometric coefficient which provides information about moles of reactants & products.

One mole of nitrogen (N2) reacts with 3 moles of hydrogen (H2) to give 2 moles ammonia (NH3).

28 grams of nitrogen reacts with 6 grams of hydrogen to give 34 grams of ammonia.

Limiting reagent-

Sometimes reactions are carried out with the amounts of reactants that are different than the amounts required in a balanced chemical equation. The reactant which is present in the least amount gets consumed earlier and after that reaction does not takes place. Therefore the reactant which gets consumed first, limits the amount of products formed, hence called limiting reagent.

Ex-The given chemical reaction is initiated by passing spark through a reaction vessel containing 12 moles of H2 & 8 moles of O2,

2H2(g) + O2(g) —> 2H2O(g)

Initial Moles                       12              8                     0

Moles After reaction         0               8-6 = 2         6 x 2=12

The reaction stops after consumption of 6 moles O2 because H2 is consumed completely. Therefore H2 is limiting reagent, it limits the amount of product.

The other reactant which is present in excess is called an excess reactant.