Basic Stoichiometry and Calculations

Stoichiometry: Basics and Calculations

Stoichiometry can seem daunting to the beginning chemistry student, but is perhaps one of the most basic and simple concepts in any chemistry course. Stoichiometry refers to the fixed ratios in which substances react, and the analysis that is enabled through the knowledge of those ratios. In this tutorial, I will assume the reader is familiar with chemical equations.

Section 1.0 – The Basics

It was previously mentioned that stoichiometry refers to the ratios in which substances react. A great way to delve into stoichiometry is by considering a simple reaction equation. Consider the reaction in which hydrogen combusts to form water:

2H2 + O2 -> 2H2O

Here, two units of hydrogen (H2) react with one unit of oxygen (O2) to form two units of water (H2O). It then follows that for each unit of hydrogen (H2) that combusts, one unit of water (H2O) forms. This is an example of a fixed ratio in a chemical reaction. Now consider another reaction, where sodium and chlorine combine to form sodium chloride:

Na + Cl -> NaCl

In this case, the reaction is very simple. One unit of sodium chloride is formed from the reaction of one unit of sodium and one unit of chlorine. This means for each unit of chlorine that reacts, one unit of sodium chloride is formed. It then follows that for each unit of sodium chloride that forms, one unit of sodium reacted. This is also an example of a fixed ratio.

Section 2.0 – Calculations

The unit used in chemistry for stoichiometric calculations is the mole. (One mol = 6.02 x 10^23 elementary entities.) Now to consider a simple stoichiometric problem: How many moles of aluminium reacted to form 20 moles of aluminium iodide?

To begin, write a balanced chemical equation for the reaction:

2Al + 3I2 -> 2AlI3

Now the stoichiometric analysis begins. We know that for every 2 moles aluminium that reacted, 2 moles of aluminium iodide formed. Thus every unit (mole) of aluminium that reacts forms one unit (mole) of aluminium iodide. It then follows that in order to yield 20 moles of AlI3, 20 moles aluminium must have reacted. Now for a slightly more difficult problem: How many moles of carbon dioxide is produced when 12 moles of methane gas combusts?

Once again, write a balanced equation:

CH4 + 2O2 -> CO2 + 2H2O

Therefore, for every mole of methane that combusts, one mole of CO2 is produced. Therefore, if 12 moles of methane are combusted, 12 moles of CO2 is produced. This is the reasoning behind any basic stoichiometric calculation.

A good understanding of these basics can bring success in just about any chemistry course. Be sure to always verify reasoning, and stoichiometry will be a piece of cake.