75-Archetype SyllabusChemistry • ARCH-C-17 • Spec C3

Full 6-Card Lesson 15-Question Timed Drill

UAT-UK SPEC CODE: C3 (Chemical formulae, equations and mass fractions)ASSUMED MATHS 1 PREREQUISITE: M3.5FREQUENCY: VERY HIGH

Empirical/Molecular Formula from Reacting Mass Fractions & Hydrated Salt Water of Crystallisation (M·xH2O)

Deconstructed from 20 past UAT-UK questions (8.89% 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

20 Past Qs

8.89% 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-17.

Launch All 6 Active-Recall Cards

CARD 1 OF 6 • THE CLASSROOM TIME TRAP (150s)

Why Students Bleed Time:

Step 1: Find the mass of water driven off:

m(\text{H}_2\text{O}) = 5.00 - 3.20 = 1.80\text{ g}

. Step 2: Calculate the moles of anhydrous

\text{CuSO}_4

(

M_r = 64 + 32 + 4(16) = 160

) and moles of

\text{H}_2\text{O}

(

M_r = 18

n(\text{CuSO}_4) = \frac{3.20}{160} = 0.020\text{ mol}

and

n(\text{H}_2\text{O}) = \frac{1.80}{18} = 0.10\text{ mol}

. Step 3: Divide to find the mole ratio:

x = \frac{n(\text{H}_2\text{O})}{n(\text{CuSO}_4)} = \frac{0.10}{0.020} = 5

(~95s).

In 20 past UAT-UK questions (8.89% of Chemistry), candidates lose 110s by the mole ratio x = n(h2o) / n(m) must compare water lost against the remaining anhydrous salt (m_final / m_r(m)), never the initial hydrated mass! Instead, apply C3 with Maths 1 skill M3.5.

CARD 2 OF 6 • THE 2-LINE SUB-90 HEURISTIC (<40s)

Line 1 (Moles of Anhydrous Salt vs Water Loss):

n(\text{CuSO}_4) = \frac{3.20}{160} = 0.020\text{ mol}

;

n(\text{H}_2\text{O}) = \frac{5.00 - 3.20}{18} = \frac{1.80}{18} = 0.10\text{ mol}

Line 2 (Ratio):

x = \frac{0.10}{0.020} = 5

(20s).

Sanity & Limit Check: Verify units/signs against fence_post_or_domain_trap (In heating a hydrated salt M·xH2O, dividing mass of water lost (m_initial - m_final) by the initial HYDRATED mass instead of the final ANHYDROUS salt mass m_final) and fence_post_or_domain_trap (Rounding a 1 : 1.33 or 1 : 1.5 mole ratio down to 1 : 1 instead of multiplying all ratios by 3 (to get 3 : 4, e.g. Fe3O4) or by 2 (to get 2 : 3, e.g. Fe2O3)).

Examiner Distractor Blueprint for ARCH-C-17

FOIL #1fence_post_or_domain_trap

In heating a hydrated salt M·xH2O, dividing mass of water lost (m_initial - m_final) by the initial HYDRATED mass instead of the final ANHYDROUS salt mass m_final

The mole ratio x = n(H2O) / n(M) must compare water lost against the remaining anhydrous salt (m_final / M_r(M)), never the initial hydrated mass!

Open Fence-Post / Domain Trap Clinic

FOIL #2fence_post_or_domain_trap

Rounding a 1 : 1.33 or 1 : 1.5 mole ratio down to 1 : 1 instead of multiplying all ratios by 3 (to get 3 : 4, e.g. Fe3O4) or by 2 (to get 2 : 3, e.g. Fe2O3)

Force-rounding a .33 or .5 decimal mole ratio instead of scaling to the smallest whole-number ratio.

Open Fence-Post / Domain Trap Clinic

FOIL #3intermediate_stop_trap

Stopping at the empirical formula (e.g. CH2O, M_r = 30) without scaling by (M_r_actual / M_r_empirical) to obtain the molecular formula (e.g. C6H12O6, M_r = 180)

Reporting the simplest integer ratio when the question asks for the true molecular formula of the given M_r.

Open Intermediate Stop Trap Clinic

FOIL #4subsystem_isolation_trap

Using crucible + sample gross mass without subtracting the empty crucible tare mass before calculating moles of metal and oxygen

Failing to subtract the empty container mass from both initial and final crucible weighings.

Open Subsystem Isolation 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-17)

ARCH-C-17: Empirical/Molecular Formula from Reacting Mass Fractions & Hydrated Salt Water of Crystallisation (M·xH2O) (Sample)Chemistry

00:00.0

SUB-90 ZONE (<40S)

0.0s40s Sub-9090s ESAT Budget150s Classroom

Question 1 of 1ARCH-C-17 (6-Card Lesson)Format: Standard 5-Option

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Question 1 of 1 • 90s Target

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Chemistry•ARCH-C-17•Standard 5-Option

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