# What is stoichiometry, and how is it used in chemical reactivity?

Jun 4, 2016

You must (i) have a balanced chemical equation, and (ii) know some basic arithmetic.

#### Explanation:

Let's look at another scenario. You go into a shop, and you purchase an item whose price is £23-30. You give the clerk a £50-00 note. The clerk gives you the item and £17-70 in change. Would you know whether you had been short-changed? I think you would know, and you would have cause for complaint.

In the banking and finance caper, there is a saying, $\text{for every credit there must be a corresponding debit}$. What does this mean? It means that if you credit an account, some other account must be debited, or there must be cash on hand (cash, so I am told, is a debit item).

What applies in the banking and finance industries, applies even more so in chemistry, because there are more particles to balance. This illustrates the fundamental principle of $\text{stoichiometry}$. Mass is ALWAYS conserved. If you start with $10 \cdot g$ of reactant, at most you are going to get $10 \cdot g$ of product. In practice, you are not even going to get that, because losses invariably occur on handling. So bear this in mind the next time you balance a chemical equation: if the reactants were money, would you be ripped off? In honesty, I do not think you would be.

So (finally!) a problem. I combust $16 \cdot g$ of methane, $C {H}_{4}$, according to the following rxn:

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(l\right)$

What is the mass of oxygen gas that I need? What mass of carbon dioxide will I get? Note that mass (and atoms) are necessarily conserved in every chemical reaction.

Try the process with another example; the combustion of hexanes, ${C}_{5} {H}_{12}$. This combusts according the equation:

$\text{Pentane + oxygen "rarr" carbon dioxide + water}$

${C}_{5} {H}_{12} + x {O}_{2} \left(g\right) \rightarrow y C {O}_{2} + z {H}_{2} O$

How to balance this? Remember, if it were money you would do it automatically. Balance $C$, then $H$, and then $O$.