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UAT-UK SPEC: C9MODULE: CHEMISTRYTOPIC ID: ARCH-C-16

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

Master Multi-Step Acid-Base Back-Titrations, Diprotic Ratios (H2SO4/Ca(OH)2), Thermometric Titration & Purity on the ESAT Chemistry paper. Traditional classroom derivations waste valuable seconds. Using the 2-step Sub-90 invariant elimination method compresses solve time to 45s, saving +115s per question.

Standard Classroom Time
160s
Textbook multi-line derivations bleed exam clock
Sub-90 Target Pace
45s
Comfortably below the 88.8s per-question limit
Time Saved for Review
+115s
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 (Millimoles with 1:21:2 Ratio): n(H2SO4)=12×25.0×0.16=2.0 mmoln(\text{H}_2\text{SO}_4) = \frac{1}{2} \times 25.0 \times 0.16 = 2.0\text{ mmol}.

Establish the structural invariant and eliminate impossible options immediately.

STEP 2 • FAST SOLVE (LINE 2)15–25s
Line 2 (Divide by 20.0 cm320.0\text{ cm}^3): c=2.0 mmol20.0 cm3=0.10 mol dm−3c = \frac{2.0\text{ mmol}}{20.0\text{ cm}^3} = 0.10\text{ mol dm}^{-3} (20s).

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

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-.

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

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

Sub-90 Fix: Test invariants before expanding equations.

Try 1 Timed Question: Multi-Step Acid-Base Back-Titrations, Diprotic Ratios (H2SO4/Ca(OH)2), Thermometric Titration & Purity

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Full Lesson All 15 Questions (ARCH-C-16)
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ARCH-C-16: Multi-Step Acid-Base Back-Titrations, Diprotic Ratios (H2SO4/Ca(OH)2), Thermometric Titration & Purity (Sample)Chemistry
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SUB-90 ZONE (<40S)
0.0s40s 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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