The Chemical Recipe Book
3.2.3 | Reacting Masses in Simple Proportions
The Analogy: Chemical equations are like recipes. If a recipe says "2 eggs + 1 cup flour → 12 cookies", you can scale it up or down. Want 6 cookies? Use half the ingredients. Want 24? Double everything. Chemistry works the exact same way.
📖 Scenario: The Carbon Burning Factory
You are a chemist at a power plant. Your job is to burn carbon to produce energy. The equation is: C + O₂ → CO₂
From the periodic table, you know: C = 12, O = 16 (so O₂ = 32, CO₂ = 44). This means the "recipe" is: 12g Carbon needs 32g Oxygen to make 44g CO₂.
🔧 The Recipe Scaler
📐 The Mathematics:
Base recipe: 12g C needs 32g O₂ → produces 44g CO₂
You have: 12.0g C
Scaling factor: 12.0 ÷ 12 = 1.00
O₂ needed: 32 × 1.00 = 32.0g
CO₂ made: 44 × 1.00 = 44.0g
💡 The Key Insight
Notice how all three numbers scale together? If you halve the carbon, you halve the oxygen needed AND halve the CO₂ produced. This is called proportional scaling. The ratio stays constant: 12:32:44 is the same as 6:16:22 or 3:8:11.
⚠️ EXAMINER TRAP ALERT
Trap #1: Students forget to balance the equation first. An unbalanced equation gives wrong ratios.
Trap #2: Students confuse "reacting mass" with "Mr". One is the actual mass you use, the other is just the formula mass.
Trap #3: Students don't show their working. Even if your final answer is right, you might lose marks for not showing the ratio step.
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