Chemistry · 35 minutes
Suggested grades 10–11 · Unit 2: Reactions and Energy · Lesson 1 of 2
Scale a Reaction Without Creating Atoms
Use coefficients and proportional amounts in an idealized reaction model.
Mr. S Teaches · Original starter lesson · HS-CHEM-03 · Grade range is planning guidance.Watch, notice, discuss
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Stoichiometry - Chemistry for Massive Creatures: Crash Course Chemistry #6
By CrashCourse
Pause at mole ratios and compare coefficients with the lesson’s conserved atom account.
Open on YouTube: Stoichiometry - Chemistry for Massive Creatures: Crash Course Chemistry #6 (opens a new tab) ↗Stoichiometry Tutorial: Step by Step Video + review problems explained | Crash Chemistry Academy
By Crash Chemistry Academy
Preview the worked calculations as an alternative explanation of reaction scaling; adapt numbers to the supplied limiting amount.
Open on YouTube: Stoichiometry Tutorial: Step by Step Video + review problems explained | Crash Chemistry Academy (opens a new tab) ↗What idea from the video helps you calculate a product amount from a limiting supplied reactant and explain the conserved atom counts? Identify one useful example and one limit of the video's model or explanation.
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Learning objective
Calculate a product amount from a limiting supplied reactant and explain the conserved atom counts.
Before you begin
- Read formulas and multiply ratios.
Gather your materials
- Paper or notebook
- Pencil; calculator optional for numerical work; all task data supplied below
Language to know
- coefficient
- A multiplier stating relative numbers of particles or moles in an equation.
- mole
- A counting unit for an amount of substance.
- limiting reactant
- The reactant used up first under the stated reaction model.
Understand the concept
Paper reaction account: 2H2 + O2 → 2H2O. Coefficients give ratios of molecules or moles, not grams. With 6 mol H2 and 2 mol O2, oxygen can react with 4 mol H2 and produce 4 mol H2O, leaving 2 mol H2. Each scaled batch preserves atom counts. This is an accounting exercise only; do not combine hydrogen and oxygen. Actual yield can differ if reactions are incomplete or competing processes occur.
Teach it step by step
- Read the equation and count atoms in one coefficient-sized batch.
- Draw two such batches to match 2 mol O2 and track the required H2 amount.
- Calculate product and leftover reactant using the same ratio.
- Explain the result in words and distinguish mole ratios from mass ratios.
Make the thinking visible
Worked example or model response
One batch uses 2 mol H2 for 1 mol O2. Two batches use 4 and 2 respectively, producing 4 mol water. The remaining 2 mol H2 cannot form more water without additional O2 in this model.
Guided practice
Together find the product when 3 mol H2 and 3 mol O2 are supplied. Identify the limiting reactant before subtracting leftovers.
Independent practice
Solve the model for 10 mol H2 and 4 mol O2. State product, leftover amounts and atom-conservation reasoning. Explain why changing H2O to H2O2 is not a valid balancing method.
Quick assessment
Can equal coefficients be read automatically as equal gram masses?
Answer key & assessment criteria
Answers and teaching guidance
- Guided: H2 limits; 3 mol H2 reacts with 1.5 mol O2 to give 3 mol H2O, leaving 1.5 mol O2.
- Independent: O2 limits; 8 mol H2 reacts with 4 mol O2 to produce 8 mol H2O, leaving 2 mol H2. Changing a subscript changes the substance rather than the amount.
- Quick check: No. Coefficients describe relative counts or moles; masses depend on the substances’ molar masses.
Look for: Assess the stated objective using the concrete criteria and explanations in the answer key. Invite an alternative supported solution; distinguish a model’s prediction from an established observation.
Make it work for your learner
More support
Use grouped paper tokens and a ratio table before introducing the word mole.
Extension challenge
Using supplied molar masses H2=2 g/mol, O2=32 g/mol and H2O=18 g/mol, verify mass conservation for the independent example including leftovers.
Verified references · Selected contributions
Standards connections
Selected NGSS high school-band contribution; full performance requires additional evidence and instruction.
Subject-content standards
These records identify the part of a subject standard practiced here. Course placement is instructional guidance; the source’s category or band remains authoritative.
Contributes to NGSS performance expectation
HS-PS1-7
NGSS · High school band
Use mathematical representations of conserved atoms and mass in reactions.
Read the official performance expectation ↗Selected reaction ratios and conservation calculations are modeled without a laboratory reaction. Real yields, measurement uncertainty and a wider range of reaction systems require further work.
Verified 2026-11-04. Selected contribution, not full attainment.
Supporting literacy connections
These records apply to the specified reading, writing or communication task. They do not establish subject-content alignment.
Supporting Common Core literacy connection
RST.9-10.7
Common Core · Grades 9–10
Translate technical prose into visuals and explain mathematical or visual information.
Read the official standard ↗The activity explicitly analyzes the provided technical text, table or model. It contributes selected literacy practice rather than complete grade-band reading coverage or independent subject-content alignment.
Verified 2026-11-04. Selected contribution, not full attainment.