Introduction To Stoichiometry

Mass Relations and Balancing Equations

You need to understand stoichiometry to predict what happens when you mix chemicals.
You need to understand stoichiometry to predict what happens when you mix chemicals. Steve McAlister, Getty Images

One of the most important parts of chemistry is stoichiometry. Stoichiometry is the study of the quantities of reactants and products in a chemical reaction. The word comes from the Greek words: stoicheion ("element") and metron ("measure"). Sometimes you'll see stoichiometry covered by another name: Mass Relations. It's a more easily pronounced way of saying the same thing.

Stoichiometry Basics

Mass relations are based on three important laws. If you keep these laws in mind, you'll be able to make valid predictions and calculations for a chemical reaction.

  • Law of Conservation of Mass - mass of the products equals the mass of the reactants
  • Law of Multiple Proportions - the mass of one element combines with a fixed mass of another element in a ratio of whole numbers
  • Law of Constant Composition - all samples of a given chemical compound have the same elemental composition

Common Stoichiometry Concepts and Problems

The quantities in stoichiometry problems are expressed in atoms, grams, moles, and units of volume, which means you need to be comfortable with unit conversions and basic math. To work mass-mass relations, you need to know how to write and balance chemical equations. You'll need a calculator and a periodic table.

Here's information you need to understand before you start work with stoichiometry:

A typical problem gives you an equation, asks you to balance it, and to determine the amount of reactant or product under certain conditions. For example, you may be given the following chemical equation:

2 A + 2 B → 3 C

and asked, if you have 15 grams of A, how much C can you expect from the reaction if it goes to completion? This would a be a mass-mass question. Other typical problem types are molar ratios, limiting reactant, and theoretical yield calculations.

Why Stoichiometry Is Important

You can't understand chemistry without grasping the basics of stoichiometry because it helps you predict how much of a reactant participates in a chemical reaction, how much product you'll get, and how much reactant might be left over.

Tutorials and Worked Example Problems

From here, you can explore specific stoichiometry topics:

Quiz Yourself

Do you think you understand stoichiometry. Test yourself with this quick quiz.