Stoichiometry is one of the most important subjects in general chemistry. It is typically introduced after discussing parts of the atom and unit conversions. While it's not difficult, many students get put off by the complicated-sounding word. For this reason, it may be introduced as "Mass Relations."
Stoichiometry is the study of the quantitative relationships or ratios between two or more substances undergoing a physical change or chemical change (chemical reaction). The word derives from the Greek words: stoicheion (meaning "element") and metron (meaning "to measure"). Most often, stoichiometry calculations deal with the mass or volumes of products and reactants.
Pronounce stoichiometry as "stoy-kee-ah-met-tree" or abbreviate it as "stoyk."
What Is Stoichiometry?
Jeremias Benjaim Richter defined stoichiometry in 1792 as the science of measuring quantities or mass ratios of chemical elements. You might be given a chemical equation and the mass of one reactant or product and asked to determine the quantity of another reactant or product in the equation. Or, you might be given the quantities of reactants and products and asked to write the balanced equation that fits the math.
Important Concepts in Stoichiometry
You must master the following chemistry concepts to solve stoichiometry problems:
Remember, stoichiometry is the study of mass relations. To master it, you need to be comfortable with unit conversions and balancing equations. From there, the focus is on mole relationships between reactants and products in a chemical reaction.
Mass-Mass Stoichiometry Problem
One of the most common types of chemistry problems you'll use stoichiometry to solve is the mass-mass problem. Here are the steps to solve a mass-mass problem:
- Correctly identify the problem as a mass-mass problem. Usually you're given a chemical equation, like:
A + 2B → C
Most often, the question is a word problem, such as:
Assume 10.0 grams of A reacts completely with B. How many grams of C will be produced?
- Balance the chemical equation. Make certain you have the same number of each type of atom on both the reactants and products side of the arrow in the equation. In other words, apply the Law of Conservation of Mass.
- Convert any mass values in the problem into moles. Use the molar mass to do this.
- Use molar proportion to determine unknown quantities of moles. Do this by setting two molar ratios equal to each other, with the unknown as the only value to solve.
- Convert the mole value you just found into mass, using the molar mass of that substance.
Excess Reactant, Limiting Reactant, and Theoretical Yield
Because atoms, molecules, and ions react with each other according to molar ratios, you'll also encounter stoichiometry problems that ask you to identify the limiting reactant or any reactant that is present in excess. Once you know how many moles of each reactant you have, you compare this ratio to the ratio required to complete the reaction. The limiting reactant would be used up before the other reactant, while the excess reactant would be the one leftover after the reaction proceeded.
Since the limiting reactant defines exactly how much of each reactant actually participates in a reaction, stoichiometry is used to determine theoretical yield. This is how much product can be formed if the reaction uses all of the limiting reactant and proceeds to completion. The value is determined using the molar ratio between the amount of limiting reactant and product.