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UAT-UK SPEC: C3MODULE: CHEMISTRYTOPIC ID: ARCH-C-17

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

Master Empirical/Molecular Formula from Reacting Mass Fractions & Hydrated Salt Water of Crystallisation (M·xH2O) 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 (Moles of Anhydrous Salt vs Water Loss): n(CuSO4)=3.20160=0.020 moln(\text{CuSO}_4) = \frac{3.20}{160} = 0.020\text{ mol}; n(H2O)=5.00−3.2018=1.8018=0.10 moln(\text{H}_2\text{O}) = \frac{5.00 - 3.20}{18} = \frac{1.80}{18} = 0.10\text{ mol}.

Establish the structural invariant and eliminate impossible options immediately.

STEP 2 • FAST SOLVE (LINE 2)15–25s
Line 2 (Ratio): x=0.100.020=5x = \frac{0.10}{0.020} = 5 (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 #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) / Mᵣ(M)), never the initial hydrated mass!

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

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

Stopping at the empirical formula (e.g. CH2O, Mᵣ = 30) without scaling by (M(r, actual) / M(r, empirical)) to obtain the molecular formula (e.g. C6H12O6, Mᵣ = 180)

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

Sub-90 Fix: Test invariants before expanding equations.

Try 1 Timed Question: Empirical/Molecular Formula from Reacting Mass Fractions & Hydrated Salt Water of Crystallisation (M·xH2O)

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Full Lesson All 15 Questions (ARCH-C-17)
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ARCH-C-17: Empirical/Molecular Formula from Reacting Mass Fractions & Hydrated Salt Water of Crystallisation (M·xH2O) (Sample)Chemistry
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SUB-90 ZONE (<35S)
0.0s35s 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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