Introduction to Stoichiometry

Overview

What is Stoichiometry?

Stoichiometry is at the heart of the production of many things you use in your daily life. Soap, tires, fertilizer, gasoline, deodorant, and chocolate bars are just a few commodities you use that are chemically engineered, or produced through chemical reactions. Chemically engineered commodities all rely on stoichiometry for their production.

But what is stoichiometry? Stoichiometry is the calculation of quantities in chemical equations. Given a chemical reaction, stoichiometry tells us what quantity of each reactant we need in order to get enough of our desired product. Because of its real-life applications in chemical engineering as well as research, stoichiometry is one of the most important and fundamental topics in chemistry.

Introduction to the Mole

Which weighs more, 100 pounds of feathers or 100 pounds of bowling balls? You've probably heard this riddle before. Obviously they both weigh the same since I told you I have 100 pounds of each. But if I have 100 pounds of bowling balls and 100 pounds of feathers, do I have more feathers or more bowling balls? The quantities of feathers and bowling balls would not be equal. An individual feather weighs a lot less than a bowling ball. It would take only about 10 bowling balls to get 100 pounds whereas it would take a LOT more feathers.

When you measure quantities in moles, however, the situation is exactly opposite from when you measure according to weight. A mole is defined as the amount of a substance. More specifically, there are 6.02×10²³ particles in a mole of substance. Therefore, if you had 1 mole of feathers and 1 mole of bowling balls, you would have 6.02×10²³ feathers and 6.02×10²³ bowling balls. Now suppose you were asked the question, "Which weighs more, 100 moles of feathers or 100 moles of bowling balls?" The answer this time would be the bowling balls. Although there is an equal number of both feathers and bowling balls, an individual bowling ball weighs much more than an individual feather, and so an equal number of bowling balls would weigh more than an equal number of feathers.

Now, let's return to the number 6.02×10²³ . This number is known as Avogadro's number and you should definitely commit it to memory. You are probably wondering why it's so large, and it does indeed look intimidating without the exponential notation:

Although you will never have a mole of bowling balls, you will soon be calculating moles of compounds, molecules, atoms, and ions. These representative particles are extremely and incredibly small. That is why there are so many particles in a mole of substance. When you appreciate just how small these particles are, 6.02×10²³ stops seeming like such a crazy number.