AP Chemistry Stoichiometry: Zero to 5 in Two Weeks

The Golden Rule: The Mole is the Bridge

Every stoichiometry problem involves converting between mass, moles, and particles. The mole (mol) is always the unit you pass through. Commit this roadmap to memory:

• Mass (grams) → Moles: divide by molar mass (g/mol)
• Moles → Mass: multiply by molar mass
• Moles → Particles: multiply by Avogadro's number (6.022 × 10²³)
• Moles of A → Moles of B: use the mole ratio from the balanced equation

Step-by-Step Framework (Works for Every Problem)

1. 1Write and balance the chemical equation
2. 2Identify what you are given (with units) and what you need to find
3. 3Convert given quantity to moles (if not already in moles)
4. 4Use the mole ratio from the balanced equation to convert to moles of the target species
5. 5Convert moles of target to the desired unit (grams, liters, particles)
6. 6Check: does the magnitude make sense? Do units cancel correctly?

Limiting Reagent Problems

When you are given amounts of two or more reactants, you must find the limiting reagent — the one that runs out first and determines the maximum yield.

1. 1Convert both reactant masses to moles
2. 2Divide each moles value by its stoichiometric coefficient from the balanced equation
3. 3The reactant with the SMALLER result is the limiting reagent
4. 4Use only the limiting reagent amount to calculate moles of product
5. 5The reagent with the larger result is in excess — calculate how much excess remains if needed

Percent Yield

In real reactions, you never recover 100% of your theoretical yield due to side reactions, incomplete reactions, and losses during purification.

• Theoretical yield: what the stoichiometry predicts you should get (calculated from limiting reagent)
• Actual yield: what you actually measured in the lab
• Percent yield = (Actual yield / Theoretical yield) × 100%
• Percent yield > 100% is not possible — if you get this, your actual yield measurement or stoichiometry is wrong

Solution Stoichiometry

When reactants are in solution, you use molarity (mol/L) to find moles: moles = Molarity × Volume (in liters).

Gas Stoichiometry

At STP (0°C, 1 atm), 1 mole of any ideal gas occupies 22.4 L. For non-STP conditions, use the ideal gas law: PV = nRT, where R = 0.0821 L·atm/mol·K.

Practice Problem Set (Difficulty-Ordered)

1. 1[Easy] How many grams of CO₂ are produced when 44 g of C₃H₈ (propane) is burned completely? (Answer: 132 g)
2. 2[Medium] 10.0 g of Al reacts with excess HCl. How many liters of H₂ gas are produced at STP? (Answer: 8.31 L)
3. 3[Hard] 25.0 mL of 0.400 M HCl reacts with 20.0 mL of 0.300 M NaOH. Which is in excess and by how many moles? (Answer: HCl in excess by 0.004 mol)
4. 4[Hard] A reaction gives 18.5 g of product. The theoretical yield is 23.2 g. What is the percent yield? (Answer: 79.7%)