Stoichiometry

Grade 11 · Science · Study Guide

Learn how chemists use balanced equations and the mole concept to predict exactly how much of each substance reacts or forms in a chemical reaction.

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. At its core, stoichiometry relies on one foundational idea: atoms are conserved. Whatever atoms enter a reaction must come out on the other side, just rearranged into new compounds. This is why we balance chemical equations — a balanced equation is essentially a recipe that tells you the exact ratio of particles involved.

The mole is the central unit of stoichiometry. One mole equals 6.022 × 10²³ particles (Avogadro's number). Because individual atoms and molecules are far too small to count directly, chemists work in moles and convert to mass using molar mass — the mass in grams of one mole of a substance, numerically equal to its atomic or molecular mass in atomic mass units. For example, one mole of water (H₂O) has a molar mass of 18.02 g/mol.

Mole ratios are the heart of stoichiometric calculations. Once an equation is balanced, the coefficients give you the mole ratio between any two substances. In the reaction 2H₂ + O₂ → 2H₂O, the ratio of H₂ to O₂ is 2:1, meaning 2 moles of hydrogen react with exactly 1 mole of oxygen. You use this ratio as a conversion factor to find how many moles or grams of any substance you need or will produce.

Limiting reagents add a practical layer to stoichiometry. The limiting reagent is the reactant that runs out first and limits how much product forms. The excess reagent is whatever is left over. Identifying the limiting reagent requires calculating the moles available for each reactant and comparing them to what the ratio demands.

Percent yield connects theoretical predictions to experimental reality. Theoretical yield is the maximum amount of product calculated from stoichiometry. Actual yield is what you collect in the lab. Percent yield = (actual yield / theoretical yield) × 100. A yield of 85-95% is considered good in most laboratory settings.

Key terms

Mole

The SI unit for amount of substance; 1 mol = 6.022 × 10²³ particles.

Molar Mass

The mass in grams of one mole of a substance, equal to its formula mass in g/mol.

Mole Ratio

The ratio of moles of any two substances in a balanced equation, derived from coefficients.

Limiting Reagent

The reactant completely consumed first, determining the maximum product formed.

Excess Reagent

The reactant present in more than the stoichiometrically required amount.

Theoretical Yield

Maximum mass of product predicted by stoichiometric calculation from the limiting reagent.

Percent Yield

(Actual yield / Theoretical yield) × 100%.

Avogadro's Number

6.022 × 10²³ — the number of particles in one mole of a substance.

Practice questions

1. In N₂ + 3H₂ → 2NH₃, how many moles of NH₃ are produced from 4 moles of H₂?

   1. 1.33 mol
   2. 2.67 mol
   3. 4.00 mol
   4. 6.00 mol
   Show answer

   Mole ratio NH₃:H₂ = 2:3. So 4 mol H₂ × (2/3) = 2.67 mol NH₃.

2. What is the molar mass of Ca(OH)₂?

   1. 57.09 g/mol
   2. 74.10 g/mol
   3. 56.08 g/mol
   4. 78.12 g/mol
   Show answer

   Ca=40.08, O×2=32.00, H×2=2.02. Total=74.10 g/mol.

3. The reactant completely consumed in a reaction, limiting the amount of product formed, is called the ______.

   Show answer

   The limiting reagent determines theoretical yield because it runs out first.

4. A student gets 4.5 g of product; theoretical yield was 5.0 g. What is percent yield?

   1. 80%
   2. 85%
   3. 90%
   4. 95%
   Show answer

   (4.5 / 5.0) × 100 = 90%.

5. How many molecules are in 2.0 moles of CO₂?

   1. 3.01 × 10²³
   2. 6.02 × 10²³
   3. 1.20 × 10²⁴
   4. 2.41 × 10²⁴
   Show answer

   2.0 mol × 6.022 × 10²³ = 1.20 × 10²⁴ molecules.

Fun facts

• Avogadro's number is so large that if you had a mole of seconds, it would be about 19 quadrillion years — roughly 1,400 times the age of the universe.
• Industrial stoichiometry drives the Haber process (N₂ + 3H₂ → 2NH₃), producing fertilizer for roughly half the world's food supply.
• Percent yields in pharmaceutical manufacturing are tracked to fractions of a percent — a 1% improvement for a drug made at ton scale can mean millions in savings annually.

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