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75-Archetype SyllabusChemistry • ARCH-C-03 • Spec C4
Full 6-Card Lesson 15-Question Timed Drill
UAT-UK SPEC CODE: C4 (Quantitative chemistry)ASSUMED MATHS 1 PREREQUISITE: M3.5FREQUENCY: MEDIUM

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

Deconstructed from 8 past UAT-UK questions (3.56% of all historical Chemistry papers). Students using standard A-Level classroom methods spend an average of 150s on this archetype—blowing past the 90s ESAT budget. Using the 2-line Sub-90 heuristic below compresses solve time to 40s (saving 110s per question).

Historical Frequency
8 Past Qs
3.56% of Chemistry
Classroom Trap Time
150s
+60s over 90s budget
Sub-90 Target Time
40s
Banks +110s on the clock
Dominant UI Format
5-Option A–E
Maths 1 Skill: M3.5

Card 1 & Card 2 Preview: The Classroom Time Trap vs. Sub-90 2-Line Rule

Excerpted from our interactive 6-Step Topic Lesson for ARCH-C-03.

Launch All 6 Active-Recall Cards
CARD 1 OF 6 • THE CLASSROOM TIME TRAP (150s)
Why Students Bleed Time:
Write the balanced equation: C3H8(g)+5O2(g)→3CO2(g)+4H2O(l)\text{C}_3\text{H}_8\text{(g)} + 5\text{O}_2\text{(g)} \rightarrow 3\text{CO}_2\text{(g)} + 4\text{H}_2\text{O(l)}. By Avogadro's Law, 20 cm320\text{ cm}^3 of C3H8\text{C}_3\text{H}_8 reacts with 5×20=100 cm35 \times 20 = 100\text{ cm}^3 of O2\text{O}_2 to form 3×20=60 cm33 \times 20 = 60\text{ cm}^3 of CO2\text{CO}_2. The unreacted excess O2\text{O}_2 is 150−100=50 cm3150 - 100 = 50\text{ cm}^3. At rtp, H2O\text{H}_2\text{O} is liquid, so total gas =60+50=110 cm3= 60 + 50 = 110\text{ cm}^3 (~125s).
In 8 past UAT-UK questions (3.56% of Chemistry), candidates lose 110s by using atomic mass 16 for o₂ gas instead of molecular mass 32. Instead, apply C4 with Maths 1 skill M3.5.
CARD 2 OF 6 • THE 2-LINE SUB-90 HEURISTIC (<40s)
**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.
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).
Sanity & Limit Check: Verify units/signs against factor_of_two_or_stoichiometry_trap (Dividing mass by atomic number Z instead of relative atomic mass A_r, or forgetting diatomic O₂ / H₂ molar mass (32 / 2)) and subsystem_isolation_trap (Including condensed liquid H_2O volume in final rtp gas volume after cooling below 100 °C).

Examiner Distractor Blueprint for ARCH-C-03

FOIL #1factor_of_two_or_stoichiometry_trap
Dividing mass by atomic number Z instead of relative atomic mass A_r, or forgetting diatomic O₂ / H₂ molar mass (32 / 2)

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

Open Factor-of-2 / Stoichiometry Clinic
FOIL #2subsystem_isolation_trap
Including condensed liquid H_2O 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 + CO_2 are gases.

Open Subsystem Isolation Trap Clinic
FOIL #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.

Open Fence-Post / Domain Trap Clinic
FOIL #4intermediate_stop_trap
Outputting CO_2 volume formed instead of total final gas mixture volume (V_CO2 + V_O2_unreacted)

Forgetting to add the excess unreacted oxygen remaining in the vessel.

Open Intermediate Stop Trap Clinic

Try 1 Live Sample Question Under the 90-Second Stopwatch

Press W to open the 2-line dry-wipe scratchpad, select A–E, and lock in to reveal the Dual-View solution.

Full 6-Card Lesson Launch 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
00:00.0
SUB-90 ZONE (<40S)
0.0s40s Sub-9090s ESAT Budget150s Classroom
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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The question stem, diagram, and answer options are hidden behind this pre-start gate. When you click below, the question will reveal and your 90-second split stopwatch will immediately begin ticking.

Chemistry•ARCH-C-03•Standard 5-Option
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