# How is a limiting reactant determined?

Dec 22, 2016

The first requirement is a $\text{stoichiometrically balanced equation....}$

#### Explanation:

And what does $\text{stoichiometric}$ mean? It means balanced with respect to mass. If you start with 10 g of reactant from all sources, AT MOST you can get 10 g of product. And in practice, since losses always occur on handling, you are not even going to get that.

And not only is a reaction balanced with respect to mass, it is also balanced with respect to charge. Generally, we start with neutral reactants; neutral products MUST be obtained. Of course, if we articificially split up oxidation and reduction reactions, net electric charge must always be conserved from reactants to products.

The principle that mass and charge are conserved in a chemical reaction has been verified in every chemical equation ever performed. This is why educators insist that students balance chemical equations correctly.

So to illustrate this, let's take hydrocarbon combustion, the which drives our modern society. I have a quantity of methane, $10 \cdot g$, which I burn in air:

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

Is this balanced? How do you know? You should reasonably be able to tell me the limiting reagent. Why don't I need to know the mass of oxygen gas? How many grams of carbon dioxide and water will I recover?