Stoichiometric calculations are important for showing that mass is always conserved in chemical reactions.

When we look at a balanced chemical equation, we can see that the total mass of the substances we start with (called reactants) is the same as the total mass of the substances we end up with (called products).

Let’s look at an example:
Imagine the reaction:

$2H_2 + O_2 \rightarrow 2H_2O$

• Mass of Reactants:

  • 2 molecules of $H_2$ (which weighs 2 grams per mole) = 4 grams
  • 1 molecule of $O_2$ (which weighs 32 grams per mole) = 32 grams
  • Total = 4 g + 32 g = 36 g

• Mass of Products:

  • 2 molecules of $H_2O$ (which weighs 18 grams per mole) = 36 grams

As you can see, both sides equal the same amount, showing that mass is conserved.

Stoichiometric calculations help prove that in chemical reactions, nothing is created and nothing is lost!