Stoichiometry & Limiting Reagent

🧪 Stoichiometry Basics

Stoichiometry is the determination of how much product you can make from a given amount of reactant, based on the law of conservation of mass

Etymology: Greek origins - stoicheion = elements, metron = measuring

Cheeseburger Analogy 🍔

The reaction demonstrates stoichiometric ratios:

  • 2 slices of bread (B) + 1 cheese (C) + 1 patty (P) = 1 cheeseburger

  • To make 30 cheeseburgers: need 60 bread slices, 30 cheese, 30 patties

  • From 25 bread slices: can make 12 complete cheeseburgers (25 ÷ 2 = 12.5, limited by whole cheeseburgers)

📊 Molar Ratios in Chemical Equations

Molar Ratio: The coefficients from a balanced chemical equation representing the mole relationships between reactants and products

Example:

  • Molar ratio = 1:3:2

Three-Step Mass-to-Mass Conversion Process

  1. Convert grams of known substance to moles using molar mass

  2. Convert moles of known to moles of unknown using coefficients from balanced equation

  3. Convert moles of unknown to grams using molar mass

Conversion Formula

The general equation for mass-to-mass conversions:

Where X and Y are coefficients from the balanced equation

🧮 Sample Problems

Sample Problem 1

Reaction:

Question: To make 150 g of ammonium sulfate, how much ammonia is needed?

Solution:

Sample Problem 2

Reaction:

Question: With 500 g of ammonia, how much sulfuric acid is needed?

Solution:

✏ Practice Problems

Practice Problem 1

Reaction:

Question: To make 275 g of , how many grams of are produced?

Solution:

  • Substance A: (given)

  • Substance B: (solve for)

Warm-Up Calculations

  1. Molar Masses:

    • : 149.09 g/mol

    • : 142.02 g/mol

  2. Mass Conversion:

The Mole Concept and Stoichiometry 🧪

The mole serves as the fundamental bridge between microscopic particles and macroscopic measurements. This wheel diagram illustrates the three key conversions: moles ↔ grams (using molar mass), moles ↔ particles (using Avogadro's number), and moles ↔ liters (for gases at STP).

Mass-to-Mass Stoichiometry

Consider the reaction:

To calculate grams of copper recovered from a given mass of aluminum:

  1. Convert grams Al → moles Al (using molar mass from periodic table)

  2. Use mole ratio from balanced equation: 2 mol Al : 3 mol Cu

  3. Convert moles Cu → grams Cu (using molar mass)

Limiting Reagent Concept

When one reactant is completely consumed, the reaction stops. This reactant is the limiting reagent.

To identify the limiting reagent:

  1. Calculate product amount possible from first reactant

  2. Calculate product amount possible from second reactant

  3. The reactant producing less product is limiting

Solution Stoichiometry with Molarity

Molarity = moles of solute per liter of solution ()

For the reaction:

Given 50.0 mL of 0.500 M CuCl₂ solution:

  • Calculate moles CuCl₂:

  • Use mole ratio:

  • Convert to grams Cu using molar mass

Predicting Limiting Reactants

For:

Balloon

HCl (mol)

Mg (mol)

Green

0.1

0.1

Yellow

0.1

0.05

Blue

0.1

0.025

The balloon inflating most depends on moles of limiting reactant, not mass. According to the 1:2 ratio, HCl is limiting in all cases.

Key Principles

Limiting reagent determination: Based on moles present, not mass. The reactant producing fewer moles of product limits the reaction.

Excess reagent calculation: Subtract moles used (calculated from limiting reagent) from initial moles present.

Essential Vocabulary

Reactants/Reagents: Starting materials (left side of arrow)

Products: End materials (right side of arrow)

Limiting reagent: Reactant that runs out first, stopping the reaction

Excess reagent: Reactant remaining after reaction stops

Multi-Step Calculations

Stoichiometry now involves:

  • Mole ratios from balanced equations

  • Molarity as concentration ratio: