Reaction stoichiometry is an essential concept in Chemistry, focusing on the quantitative relationships between reactants and products in a chemical reaction. This serves as a brief guide for you to start solving problems on reaction stoichiometry.

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Make sure to balance your chemical equation

Chemical reactions are represented as balanced chemical equations, ensuring the same number of each type of atom on both sides. For example, the chemical equation for the combustion of methane (CH₄) is:

CH₄ + O₂ → CO₂ + H₂O

However, take note that this equation is unbalanced; it merely depicts the identity of the reactants and the products, but it is not yet useful for calculations. To be of use in stoichiometry, the reaction must be balanced.

Balancing a chemical equation is easy! You just have to make sure that the number of atoms of every element is the same on the left (reactants) and right (products) of the equation. Let’s go back to the example chemical reaction:

CH₄ + O₂ → CO₂ + H₂O

Let’s start with carbon (C). There is one atom of C on the left (in CH₄), and there is also one atom of C on the right (in CO₂). We can then say that the equation is balanced in terms of carbon, but we need to balance all the other elements as well.

Look at hydrogen (H). There are 4 atoms of H on the left (in CH₄), but only 2 atoms of H on the right (in H₂O). How do we reconcile that difference? We cannot directly add H on the right side of the equation because that will change the reaction. What we can do is strategically multiply integers to compounds so that we get the number of atoms we want. So, if we want 4 atoms of H on each side of the equation, simply multiply H₂O by 2.

CH₄ + O₂ → CO₂ + 2H₂O

2 molecules of H₂O means that you now have 4 atoms of H on the product side!

Now C and H are already balanced. There is only one element remaining: oxygen (O). Counting the atoms of O from both sides, we get 2 on the left (in O₂) and total of 4 on the right (2 from CO₂ and 2 from 2H₂O). Since we need 4 atoms of O on both sides, we simply multiply O₂ in the reactant side by 2 to balance the entire equation.

CH₄ + 2O₂ → CO₂ + 2H₂O

There we have it, our balanced equation. Do a quick count of the atoms on both sides to verify:

Atoms in Reactants	Element	Atoms in Products
1	C	1
4	H	4
4	O	4

Never proceed with stoichiometric calculations without a balanced equation. You’ll get better at balancing equations by doing a lot of practice, and you’ll start to develop your own strategy of quickly balancing the equations just by inspection.