Stoichiometry and the Mole

Learning Objectives

Define the mole and Avogadro's number.

Calculate the molar mass of a compound.

Use stoichiometry to relate the amounts of reactants and products in a balanced chemical equation.

Determine the limiting reactant in a chemical reaction.

The Heart of Chemical Calculation: Stoichiometry

Stoichiometry is the quantitative study of the relationships between reactants and products in a chemical reaction. It is built upon the concept of the mole.

The Mole and Molar Mass

The Mole: The SI unit for the amount of a substance. One mole contains Avogadro's Number (approximately 6.022 x 10²³) of particles (atoms, molecules, ions, etc.). It's simply a convenient counting unit for chemists.

Molar Mass: The mass of one mole of a substance, expressed in grams per mole (g/mol). To find the molar mass of a compound, you sum the atomic masses of all atoms in its chemical formula (found on the periodic table).

Example: Molar Mass of H₂O

2 Hydrogen atoms: 2 x 1.01 g/mol = 2.02 g/mol

1 Oxygen atom: 1 x 16.00 g/mol = 16.00 g/mol

Total Molar Mass = 18.02 g/mol

This allows us to convert between the mass of a substance (in grams) and the amount of that substance (in moles), a crucial step in all stoichiometry problems.

Stoichiometric Calculations

A balanced chemical equation provides the mole ratio between reactants and products. The coefficients in the equation represent moles.

Consider the reaction: 2H₂ + O₂ → 2H₂O

This means: 2 moles of H₂ react with 1 mole of O₂ to produce 2 moles of H₂O.

General Steps for a Stoichiometry Problem:

1.Start with a balanced chemical equation.

2.Convert the given quantity (usually mass) of a substance into moles using its molar mass.

3.Use the mole ratio from the balanced equation to find the number of moles of the desired substance.

4.Convert the moles of the desired substance back into the desired quantity (usually mass) using its molar mass.

Limiting Reactants

In most reactions, reactants are not mixed in the exact stoichiometric ratio. One reactant will be completely consumed before the others.

Limiting Reactant (or Limiting Reagent): The reactant that is completely used up in a reaction and thus determines the maximum amount of product that can be formed.

Excess Reactant: The reactant that is left over after the reaction has stopped.

To find the limiting reactant:

1.Take each reactant and calculate how much product it could theoretically produce (in moles or grams).

2.The reactant that produces the least amount of product is the limiting reactant. The amount of product it can make is the theoretical yield.

Key Terms

Mole: The SI unit of amount of substance, defined as containing exactly 6.02214076×10²³ elementary entities (Avogadro's number).
Molar Mass: The mass of one mole of a chemical substance, expressed in g/mol. It is calculated by summing the atomic masses of all atoms in the formula.
Stoichiometry: The calculation of relative quantities of reactants and products in chemical reactions based on the balanced chemical equation.
Limiting Reactant: The reactant in a chemical reaction that is completely consumed when the reaction is completed