75-Archetype SyllabusChemistry • ARCH-C-16 • Spec C9

Full 6-Card Lesson 15-Question Timed Drill

UAT-UK SPEC CODE: C9 (Acids, bases and salts / Quantitative titrations)ASSUMED MATHS 1 PREREQUISITE: M3.5FREQUENCY: VERY HIGH

Multi-Step Acid-Base Back-Titrations, Diprotic Ratios (H2SO4/Ca(OH)2), Thermometric Titration & Purity

Deconstructed from 23 past UAT-UK questions (10.22% of all historical Chemistry papers). Students using standard A-Level classroom methods spend an average of 160s on this archetype—blowing past the 90s ESAT budget. Using the 2-line Sub-90 heuristic below compresses solve time to 45s (saving 115s per question).

Historical Frequency

23 Past Qs

10.22% of Chemistry

Classroom Trap Time

160s

+70s over 90s budget

Sub-90 Target Time

45s

Banks +115s 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-16.

Launch All 6 Active-Recall Cards

CARD 1 OF 6 • THE CLASSROOM TIME TRAP (160s)

Why Students Bleed Time:

First calculate the moles of

\text{NaOH}

25.0\text{ cm}^3

n(\text{NaOH}) = \frac{25.0}{1000} \times 0.16 = 0.0040\text{ mol}

. From the

1:2

stoichiometric ratio (

\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}

n(\text{H}_2\text{SO}_4) = \frac{0.0040}{2} = 0.0020\text{ mol}

. Dividing by the acid volume

V = 20.0\text{ cm}^3 = 0.020\text{ dm}^3

gives

c(\text{H}_2\text{SO}_4) = \frac{0.0020}{0.020} = 0.10\text{ mol dm}^{-3}

(~95s).

In 23 past UAT-UK questions (10.22% of Chemistry), candidates lose 115s by failing to balance h+ vs oh- equivalents: 1 mole of h2so4 or ca(oh)2 provides 2 equivalents of h+ or oh-. Instead, apply C9 with Maths 1 skill M3.5.

CARD 2 OF 6 • THE 2-LINE SUB-90 HEURISTIC (<45s)

**Line 1 (Millimoles with

1:2

Ratio):**

n(\text{H}_2\text{SO}_4) = \frac{1}{2} \times 25.0 \times 0.16 = 2.0\text{ mmol}

**Line 2 (Divide by

20.0\text{ cm}^3

):**

c = \frac{2.0\text{ mmol}}{20.0\text{ cm}^3} = 0.10\text{ mol dm}^{-3}

(20s).

Sanity & Limit Check: Verify units/signs against factor_of_two_or_stoichiometry_trap (Using a 1:1 mole ratio instead of 1:2 for diprotic H2SO4 + 2NaOH, Na2CO3 + 2HCl, or Ca(OH)2 + 2HCl) and subsystem_isolation_trap (Forgetting the 10x volumetric flask dilution factor (e.g. 25.0 cm³ pipette aliquot drawn from a 250 cm³ volumetric flask)).

Examiner Distractor Blueprint for ARCH-C-16

FOIL #1factor_of_two_or_stoichiometry_trap

Using a 1:1 mole ratio instead of 1:2 for diprotic H2SO4 + 2NaOH, Na2CO3 + 2HCl, or Ca(OH)2 + 2HCl

Failing to balance H+ vs OH- equivalents: 1 mole of H2SO4 or Ca(OH)2 provides 2 equivalents of H+ or OH-.

Open Factor-of-2 / Stoichiometry Clinic

FOIL #2subsystem_isolation_trap

Forgetting the 10x volumetric flask dilution factor (e.g. 25.0 cm³ pipette aliquot drawn from a 250 cm³ volumetric flask)

Computing moles in the 25 cm³ titration aliquot and forgetting to multiply by (250 / 25) = 10 to find total moles in the original flask.

Open Subsystem Isolation Trap Clinic

FOIL #3intermediate_stop_trap

In a back-titration, using the residual excess acid moles (titrated with NaOH) directly as the moles that reacted with the solid carbonate/metal

Forgetting to subtract residual moles from initial total acid moles: n_reacted = n_initial_acid - n_excess_titrated.

Open Intermediate Stop Trap Clinic

FOIL #4unit_or_prefix_mismatch

Multiplying concentration in mol dm⁻³ by volume in cm³ without dividing by 1000 (or failing to convert g dm⁻³ to mol dm⁻³ by dividing by M_r)

Omitting the ÷1000 conversion between cm³ and dm³.

Open Unit / Prefix Mismatch 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-16)

ARCH-C-16: Multi-Step Acid-Base Back-Titrations, Diprotic Ratios (H2SO4/Ca(OH)2), Thermometric Titration & Purity (Sample)Chemistry

00:00.0

SUB-90 ZONE (<45S)

0.0s45s Sub-9090s ESAT Budget150s Classroom

Question 1 of 1ARCH-C-16 (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-16•Standard 5-Option

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