Question

How can I know the relative number of moles of each substance with chemical equations?

Answer 1

When a reaction is balanced, then the coefficients of the compound or elements can be used to get an idea about the number of moles required or generated.

Complete answer:
In order to answer this question, we need to learn about stoichiometry. Stoichiometry is also referred to as the relative number of moles in a chemical equation. Stoichiometry is based on the fact that mass can neither be created nor destroyed in a chemical reaction. It helps in balancing the chemical reaction. It means that the number of atoms of each element in the reactant side should be numerically equal to the number of atoms of each element in the product side. Now, if we change the number of atoms of the compound for the sake of balancing, it is not possible as the formula of the compound would change. So, we multiply with the respective stoichiometric coefficients so that the reaction gets balanced. Let us consider the reaction:
$Na+{{O}_{2}}\to N{{a}_{2}}O$
Clearly, the reaction is not balanced as the number of atoms in the product and reactant side are unequal. With the trial and error method, we balance the chemical equation and multiply with respective constants. So the final balanced equation comes out to be:
$4Na+{{O}_{2}}\to 2N{{a}_{2}}O$
Here, the equation is balanced and now we can determine the number of moles. From the equation, we can say that when 4 moles of sodium react with 1 mole of oxygen , then 2 moles of $N{{a}_{2}}O$ is produced. This is how stoichiometry helps in determining the number of moles.

Note:
If we multiply every compound/element with the same constant k throughout, then the reaction still stays balanced and it does not affect the stoichiometry. On increasing the moles of reactants, the moles of products will increase too.