# How does stoichiometry operate in chemical reactions?

May 28, 2016

I hope to be of some help. Let's try the combustion of methane as an example of a chemical reaction.

#### Explanation:

The balanced equation for the combustion of methane is given by:

${\underbrace{C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right)}}_{\text{80 g" rarr underbrace(CO_2(g) + 2H_2O(l))_"80 g}}$

This equation is balanced stoichiometrically. What does it tell us? It tells us that $1$ $m o l$ of methane gas and $2$ $m o l$ of oxygen gas combine to form $1$ $m o l$ of carbon dioxide, and $2$ $m o l$ of water.

Because of the molar equivalence, i.e. the mass of a given mole of stuff, we could also say that $16 \cdot g$ of methane combines with $64 \cdot g$ oxygen gas, to give $44.0 \cdot g$ of carbon dioxide, and $36$ $g$ of water.

It is worth noting that here that mass is conserved in every chemical equation. You start with $80$ $g$ of reactant (which we did!) you must finish with $80$ $g$ of product, which we did. The balanced chemical equation precisely specifies this molar and mass equivalence.

Such stoichiometry is also practised in other scenarios: banking, finance, accounting, even a simple transaction at the supermarket. You buy goods worth £20-00 at the supermarket. You can present a £20-00 note to the cashier, and the transaction is stoichiometrically balanced. Alternatively, you can present a card, and the £20-00 is debited from your account, and credited to the supermarket's account - clearly a stoichiometric transaction, because credit item equal debit item.

This same stoichiometry operates in every chemical equation.

$\text{Garbage in EQUALS GARBAGE OUT}$

To end, you might say to me that you don't know how to balance a hydrocarbon combustion. I can guarantee that you know how to balance a cash transaction when you go shopping. You KNOW when you have been short-changed; likewise, you know when you have received too much change (and of course you give it back!). Don't short change your chemical equations; stoichiometry always operates.