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Chemistry · 3. Quantitative chemistry

Equations, limiting reagents and yield

Connect reaction observations to balanced equations and quantitative yield.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

How does a particle equation predict measurable laboratory quantities?

Curriculum coverage

Vietnam 2018 and Cambridge: amount of substance, equations, concentration and yield.

Model scope and limitations

The quantitative model assumes pure reagents and the selected equation; side reactions require separate treatment.

Learning objectives

  • Balance equations by conserving atoms.
  • Identify the limiting reagent from amount/coefficient ratios.
  • Distinguish theoretical and actual yield.

Prerequisite knowledge

  • Atoms, molecules, moles and molar mass

Core knowledge

Atom conservation

Change coefficients in front of formulas, never the formula subscripts.

Limiting reagent

Compare n/ν for each reactant; the smallest value limits reaction extent.

Percentage yield

Yield = actual/theoretical × 100%. Values above 100% often indicate wet product or measurement error.

Worked example

For 2H₂ + O₂ → 2H₂O, 0.30 mol H₂ requires 0.15 mol O₂ and can form 0.30 mol H₂O if complete.

Misconceptions to avoid

  • Equation coefficients are mole ratios, not generally mass ratios.
  • The limiting reactant is consumed first stoichiometrically, not necessarily the one with lower mass.

Virtual experiment procedure: Reactions and stoichiometry

  1. Select a reaction with a verified equation in the Chemistry lab.
  2. Record starting quantities and convert all to moles.
  3. Predict the limiting reagent, run the simulation, then compare observations and products.

Safety and cautions

  • Mix real chemicals only under teacher supervision and an approved safety procedure.
  • No visible change alone is not sufficient proof of no reaction.

Evidence to collect

  • A unit-labelled conversion chain through moles and the equation ratio.
  • A limiting-reactant check and appropriate significant figures.
Open the virtual experiment

Diagnostic check

1. What may be changed when balancing a chemical equation?

  1. Coefficients before formulas
  2. Formula subscripts
  3. Element symbols
Show answer and explanation

Correct answer: Coefficients before formulas

Changing a subscript changes the identity of the substance.

2. Which quantity should be compared to find the limiting reagent?

  1. Amount divided by coefficient
  2. Mass in grams only
  3. Container volume only
Show answer and explanation

Correct answer: Amount divided by coefficient

The reactant with the smallest n/ν is consumed first.

3. What does a 120% yield usually suggest?

  1. Wet product or measurement error
  2. Creation of new matter
  3. Always an excellent result
Show answer and explanation

Correct answer: Wet product or measurement error

Residual solvent or impurities can inflate apparent product mass.

Application practice

1. 2H₂ + O₂ → 2H₂O. With 3 mol H₂ and 2 mol O₂, what is limiting?

  1. H₂
  2. O₂
  3. Neither
Show answer and explanation

Correct answer: H₂

n/ν: H₂ = 3/2 = 1.5; O₂ = 2/1 = 2. H₂ is limiting.

2. Using the same data, what is the maximum amount of H₂O?

  1. 3 mol
  2. 2 mol
  3. 4 mol
Show answer and explanation

Correct answer: 3 mol

H₂ and H₂O have a 2:2 ratio, so 3 mol H₂ forms 3 mol H₂O.

3. Actual yield is 2.4 mol from a theoretical 3.0 mol. What is the yield?

  1. 80%
  2. 125%
  3. 20%
Show answer and explanation

Correct answer: 80%

2.4/3.0 × 100% = 80%.

Academic references

  1. OpenStax Chemistry 2e — peer-reviewed general chemistry
  2. IUPAC Gold Book — authoritative chemical terminology
  3. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

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WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer