Chemistry is the science of change and reactions form an integral part of it. Every reaction contains a reactant which undergoes a change to become or give a product. Stoichiometry deals with the quantitative aspect of reactions. It is the study of the relation between the amounts of the reactants and products in a chemical reaction. In general, when engaging with stoichiometry, we are not concerned how the reaction is taking place but we wish to know about the amount of reactant that must be present for the reaction to take place and the amount of product that will be obtained when the reaction is successfully completed. As you can tell, stoichiometry is important when it comes to conducting a reaction to ensure that the required amounts of chemicals has been allowed to react.
Matter: neither created nor destroyed
Stoichiometry is based on a very important cornerstone of chemistry: matter can neither be created nor destroyed and it can only be transformed. The law of conservation of mass states that when any change takes place, the total mass of the reactants must be equal to the total mass of the products and any loss that takes place is in fact an error in human measurement. Suppose you are conducting a chemical reaction and you are mixing two different chemicals A and B to obtain two products C and D. If you know the amount of A and B that you had initially taken and the amount of D that has been formed, you can easily determine the amount of C that you must have obtained by simply subtracting the stoichiometric mass of product D you obtained from those of A and B.
The perfect amount
Whenever you are conducting a chemical reaction in a laboratory, it is important to aim for certain results for efficiency purposes: the reaction must be able to consume all the chemicals that you utilize and there should be no leftovers. If you are mixing the base sodium hydroxide and hydrochloric acid to obtain the salt sodium chloride and water, you should know how much of the base and acid must be taken so that neither chemical remains at the end of the reaction and we only get salt and water. Stoichiometry helps you determine the perfect amount of each that must be taken to achieve the required effect.
Stoichiometry is most popularly employed to balance a chemical reaction. What does balancing a chemical reaction mean? Let us suppose that you observe three different chemicals reacting with each other to give two products and you even know the names and chemical formulae of the chemicals. In order to write a chemical reaction that is able to imply the change that took place, you will have to supplant the chemicals in the reaction with certain numbers which actually represent the number of moles of the chemical that reacted or formed. Without the use of stoichiometry, it will be difficult to understand the other aspects of the reaction such as chemical kinetics and thermodynamics.