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4.3Stoichiometry

Reaction calculations — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 3 1 quiz

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Lesson summary

Reaction calculations EST Chemistry notes: Carry out mass–mass calculations.The limiting reactant is not always the one with the smaller mass.Percent yield is below 100% due to side reactions and losses.Excess remaining = starting moles − moles used.Solution stoichiometry uses mo…

  • Theoretical yield 50.0 g, actual 42.0 g
  • Reaction calculations EST Chemistry: what to do next
  • Mole ratio
  • Excess reactant
  • Actual yield
  • Identify / Describe (FRQ)

Section 1 of 8Reaction calculations

1
Check the answer is reasonable (products cannot exceed reactant mass total).
2
Keep extra digits until the final step.
3
Coefficients give mole, not mass, ratios.
4
Titration calculations are solution stoichiometry.
5
Industrial processes aim to maximise percent yield.

Section 2 of 8Process & sequence

1
Balance the equation.
2
Convert each given amount to moles.
3
Divide moles by coefficients to find the limiting reactant.
4
Use the limiting reactant and mole ratio for the product.
5
Convert to grams and compute percent yield.

Limiting reactant methodwrite these steps in this order; each step is a marking point.

1
Balance equation
2
Moles of each reactant
3
Divide by coefficient
4
Smallest limits
5
Calculate product

Section 3 of 8Compare

Stoichiometry routes
GivenStepsWanted
Mass A÷ MA → mole ratio → × MBMass B
Moles Amole ratioMoles B
Volume gas A (STP)÷ 22.4 → mole ratioMoles B
Volume × molaritymoles → mole ratioMoles B

Section 4 of 8Worked examples

1
10.0 g H2 reacts with 32.0 g O2. Find the limiting reactant and mass of water.
  • H2: 4.96 mol ÷ 2 = 2.48; O2: 1.00 mol ÷ 1 = 1.00, so O2 limits; water = 2.00 mol = 36.0 g.
2Theoretical yield 50.0 g, actual 42.0 g.
  • Percent yield = 84.0%.

Section 5 of 8Keep going

Reaction calculations EST Chemistry: what to do next

These Reaction calculations EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Reaction calculations EST Chemistry understanding with the practice questions below, then reread any Reaction calculations EST Chemistry point you were unsure about before moving on.

1
Practice: Reaction calculations EST Chemistry practice questions
2
Previous lesson: 4.2 Formulas and equations
3
Next lesson: 5.1 Chemical equilibrium
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

Reaction calculations EST Chemistry revision notes

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Stoichiometry · Mole ratio · Limiting reactant · Excess reactant · Theoretical yield · Actual yield · Percent yield
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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