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UAT-UK SPEC: C4MODULE: CHEMISTRYTOPIC ID: ARCH-C-03

Hydrocarbon Combustion Gas Volumes (Avogadro's Law), Excess O2 & Limiting Reactants

Master Hydrocarbon Combustion Gas Volumes (Avogadro's Law), Excess O2 & Limiting Reactants on the ESAT Chemistry paper. Traditional classroom derivations waste valuable seconds. Using the 2-step Sub-90 invariant elimination method compresses solve time to 40s, saving +110s per question.

Standard Classroom Time
150s
Textbook multi-line derivations bleed exam clock
Sub-90 Target Pace
40s
Comfortably below the 88.8s per-question limit
Time Saved for Review
+110s
Banks clock buffer across 27 questions

2-Step Sub-90 Shortcut Formula

How to eliminate intermediate algebra and solve under 45 seconds.

Launch Interactive Lesson
STEP 1 • SETUP INVARIANT (LINE 1)10–15s
Line 1 (O2\text{O}_2 & CO2\text{CO}_2 Ratios): 1 C3H8:5 O2:3 CO2  ⟹  O2 left=150−100=50 cm31\text{ C}_3\text{H}_8 : 5\text{ O}_2 : 3\text{ CO}_2 \implies \text{O}_2\text{ left} = 150 - 100 = 50\text{ cm}^3; CO2=60 cm3\text{CO}_2 = 60\text{ cm}^3.

Establish the structural invariant and eliminate impossible options immediately.

STEP 2 • FAST SOLVE (LINE 2)15–25s
Line 2 (Sum Gases at rtp): 60+50=110 cm360 + 50 = 110\text{ cm}^3 (H2O(l)=0\text{H}_2\text{O(l)} = 0) (25s).

Evaluate the simplified expression using integer factoring or unit elimination.

Distractor Trap Forensics

The predictable cognitive traps engineered by examiners into the wrong options.

TRAP #1factor_of_two_or_stoichiometry_trap

Dividing mass by atomic number Z instead of relative atomic mass Aᵣ, or forgetting diatomic O₂ / H₂ molar mass (32 / 2)

Using atomic mass 16 for O₂ gas instead of molecular mass 32.

Sub-90 Fix: Test invariants before expanding equations.
TRAP #2subsystem_isolation_trap

Including condensed liquid H₂O volume in final rtp gas volume after cooling below 100 °C

Forgetting that water is a liquid of negligible volume at room temperature and pressure (rtp), whereas residual O₂ + CO₂ are gases.

Sub-90 Fix: Test invariants before expanding equations.
TRAP #3fence_post_or_domain_trap

Selecting the reactant with smaller mass/moles as limiting without dividing by stoichiometric coefficient (n / nu)

Comparing raw moles n(A) vs n(B) instead of stoichiometric equivalents n(A)/a vs n(B)/b.

Sub-90 Fix: Test invariants before expanding equations.

Try 1 Timed Question: Hydrocarbon Combustion Gas Volumes (Avogadro's Law), Excess O2 & Limiting Reactants

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Full Lesson All 15 Questions (ARCH-C-03)
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ARCH-C-03: Hydrocarbon Combustion Gas Volumes (Avogadro's Law), Excess O2 & Limiting Reactants (Sample)Chemistry
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SUB-90 ZONE (<40S)
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Question 1 of 1ARCH-C-03 (6-Card Lesson)Format: Standard 5-Option
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Question 1 of 1 • 90s Target

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