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I-09: Stoichiometry, limiting reactants and yield

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Module I-09 · Recommended before: Before reaction calculations

Stoichiometry, limiting reactants and yield

The balanced equation supplies mole ratios. It does not supply mass ratios. Limiting-reactant problems require comparing how much product each reactant could produce.

Learn the method

percent yield = actual/theoretical × 100

  1. Balance the equation.
  2. Convert given masses to moles.
  3. Use coefficient ratios.
  4. For competing reactants, calculate product from each.
  5. Choose the smaller product amount; calculate percent yield if requested.

Common mistake: Balanced coefficients are mole ratios, not gram ratios.

Worked example

Problem: For 2 H₂ + O₂ → 2 H₂O, how many moles of H₂O form from 3.0 mol H₂ with excess O₂?

Step-by-step solution: 3.0 mol H₂ × (2 mol H₂O / 2 mol H₂) = 3.0 mol H₂O.

Check the units and whether the answer is reasonable before continuing.

More applications of this mathematics

Try each extension before expanding its solution.

Limiting reactant

For 2 H₂ + O₂ → 2 H₂O, mixing 5 mol H₂ with 2 mol O₂ makes at most 4 mol H₂O. O₂ limits; 1 mol H₂ remains.

Percent yield

If the theoretical product is 10.0 g and the experiment produces 8.50 g, percent yield = (8.50/10.0)×100 = 85.0%.

Balancing and redox coefficients

For 2 Fe²⁺ → 2 Fe³⁺ + 2 e⁻, atom count and charge are conserved: the reactants have total charge +4; products have +6−2=+4.

Independent practice — five questions

Suggested mastery threshold: four out of five. Incorrect answers display a worked explanation after submission.

Choose one response for each question, then select “Check my answers.” No timer. Hints are optional. You can retry freely. Questions are formative, not proctored.

Question 1. 2 H₂ + O₂ → 2 H₂O. With excess O₂, how many moles H₂O can form from 5 mol H₂?
Hint for question 1

Write the balanced coefficient ratio.

Question 2. 2 H₂ + O₂ → 2 H₂O. With excess O₂, how many moles H₂O can form from 6 mol H₂?
Hint for question 2

Write the balanced coefficient ratio.

Question 3. 2 H₂ + O₂ → 2 H₂O. With excess O₂, how many moles H₂O can form from 7 mol H₂?
Hint for question 3

Write the balanced coefficient ratio.

Question 4. 2 H₂ + O₂ → 2 H₂O. With excess O₂, how many moles H₂O can form from 8 mol H₂?
Hint for question 4

Write the balanced coefficient ratio.

Question 5. 2 H₂ + O₂ → 2 H₂O. With excess O₂, how many moles H₂O can form from 9 mol H₂?
Hint for question 5

Write the balanced coefficient ratio.

Supporting reference

OpenStax Chemistry 2e / NIST — reference for this subject. This lesson’s wording, examples and questions are original.

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