Short on time? This page is the recap. For explanations, graphs and worked examples, use the full study guide.
Recap
- Atoms are conserved in a reaction, so a balanced equation lets you calculate any product amount from a reactant amount, or the reverse.
- The coefficients give the proportions in which particles, and therefore moles, react. They are mole ratios.
- Every calculation passes through moles. The mole ratio is the only step that crosses the equation.
- The limiting reactant runs out first and fixes the amount of product. The other reactant is in excess.
- Theoretical yield is the product calculated from the limiting reactant. Percent yield = actual ÷ theoretical × 100.
- Adding more limiting reactant gives more product (in proportion) until it stops being limiting. Adding more excess reactant gives no more product.
Key relationships
| Relationship | Units | Use it to |
|---|---|---|
| n = m / M | m in g, M in g mol⁻¹ | turn a mass into moles |
| n = PV / RT | P in atm, V in L, T in K, R = 0.08206 L atm mol⁻¹ K⁻¹ | turn gas data into moles (or V = nRT / P) |
| n = M × V | M in mol L⁻¹, V in L | turn a solution volume into moles (or V = n / M) |
| n(B) = n(A) × (coefficient of B / coefficient of A) | mol | cross the equation |
| smallest n ÷ coefficient | mol | find the limiting reactant |
| percent yield = actual / theoretical × 100 | same units top and bottom | judge how efficient the reaction was |
Assumptions behind the numbers
- The equation is balanced and is the only reaction taking place.
- The reaction goes to completion, so the limiting reactant is fully used.
- Reactants are pure and amounts are measured accurately.
- Gases behave ideally, so PV = nRT applies.
Mistakes to avoid
- Using coefficients as mass ratios. Convert to moles first.
- Comparing grams to find the limiting reactant. Compare moles ÷ coefficient.
- Ignoring coefficients. More moles does not mean “in excess” if the coefficient is larger.
- Working from the excess reactant. The product always comes from the limiting reactant.
- mL instead of L, or °C instead of K. 25.0 mL = 0.0250 L; 25 °C = 298.15 K.
- Rounding early. Keep full values until the last line.
Quick self-check
- In C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, how many moles of O₂ react with 0.20 mol of C₃H₈, and how many moles of CO₂ form? (1.0 mol O₂; 0.60 mol CO₂)
- How many moles of solute are in 30.0 mL of 0.200 M solution? (0.00600 mol)
- For A + 2B → AB₂, you mix 3.0 mol A with 4.0 mol B. Which is limiting and how much AB₂ forms? (B is limiting, 4.0 ÷ 2 = 2.0 < 3.0 ÷ 1; 2.0 mol AB₂)
- In question 3, what happens to the amount of AB₂ if you add another 1.0 mol of A? (Nothing: A is already in excess)
Next: practice questions.