75-Archetype SyllabusChemistry • ARCH-C-02 • Spec C17

Full 6-Card Lesson 15-Question Timed Drill

UAT-UK SPEC CODE: C17 (Air and water / Instrumental analysis)ASSUMED MATHS 1 PREREQUISITE: M7.7FREQUENCY: MEDIUM

Diatomic Mass Spectrometry (m/z Peak Ratios 9:6:1 & 1:2:1) & Molecular Ion Combinations

Deconstructed from 5 past UAT-UK questions (2.22% of all historical Chemistry papers). Students using standard A-Level classroom methods spend an average of 140s on this archetype—blowing past the 90s ESAT budget. Using the 2-line Sub-90 heuristic below compresses solve time to 35s (saving 105s per question).

Historical Frequency

5 Past Qs

2.22% of Chemistry

Classroom Trap Time

140s

+50s over 90s budget

Sub-90 Target Time

35s

Banks +105s on the clock

Dominant UI Format

5-Option A–E

Maths 1 Skill: M7.7

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-02.

Launch All 6 Active-Recall Cards

CARD 1 OF 6 • THE CLASSROOM TIME TRAP (140s)

Why Students Bleed Time:

Each bromine atom in

\text{Br}_2^+

has probability

0.5

of being

^{79}\text{Br}

and

0.5

of being

^{81}\text{Br}

. A molecular mass of

160

arises from either

^{79}\text{Br}-^{81}\text{Br}

(

0.5 \times 0.5 = 0.25

) or

^{81}\text{Br}-^{79}\text{Br}

(

0.5 \times 0.5 = 0.25

). Summing both arrangements gives

0.25 + 0.25 = 0.50 = 50\%

(~110s).

In 5 past UAT-UK questions (2.22% of Chemistry), candidates lose 105s by forgetting that heteronuclear diatomic ions (⁷⁹br⁸¹br)+ and (⁸¹br⁷⁹br)+ can form in 2 ordered ways (2pq). Instead, apply C17 with Maths 1 skill M7.7.

CARD 2 OF 6 • THE 2-LINE SUB-90 HEURISTIC (<35s)

Line 1 (Binomial Ratio):

(1:1)^2 \implies 1 : 2 : 1

peak heights at

m/z = 158, 160, 162

Line 2 (Middle Share):

\frac{2}{1 + 2 + 1} \times 100\% = 50\%

(25s).

Sanity & Limit Check: Verify units/signs against factor_of_two_or_stoichiometry_trap (Giving diatomic peak heights as p² : p*q : q² (1:1:1 for Br2 or 9:3:1 for Cl2) without the binomial factor of 2 on the middle peak 2pq) and fence_post_or_domain_trap (Counting m/z peak positions without checking overlapping mass sums across multi-isotope combinations).

Examiner Distractor Blueprint for ARCH-C-02

FOIL #1factor_of_two_or_stoichiometry_trap

Giving diatomic peak heights as p² : p*q : q² (1:1:1 for Br2 or 9:3:1 for Cl2) without the binomial factor of 2 on the middle peak 2pq

Forgetting that heteronuclear diatomic ions (⁷⁹Br⁸¹Br)+ and (⁸¹Br⁷⁹Br)+ can form in 2 ordered ways (2pq).

Open Factor-of-2 / Stoichiometry Clinic

FOIL #2fence_post_or_domain_trap

Counting m/z peak positions without checking overlapping mass sums across multi-isotope combinations

Treating distinct isotopic pairings with identical total mass as separate m/z peaks.

Open Fence-Post / Domain Trap Clinic

FOIL #3intermediate_stop_trap

Reporting atomic ion peaks (m/z = 35, 37) instead of molecular ion peaks (m/z = 70, 72, 74)

Confusing fragmentation monomer peaks with the parent diatomic molecular ion M+.

Open Intermediate Stop Trap Clinic

FOIL #4subsystem_isolation_trap

Dividing middle peak height by heaviest peak instead of total molecular ion intensity (p+q)²

Confusing relative peak ratio with fractional probability out of all molecular ions.

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-02)

ARCH-C-02: Diatomic Mass Spectrometry (m/z Peak Ratios 9:6:1 & 1:2:1) & Molecular Ion Combinations (Sample)Chemistry

00:00.0

SUB-90 ZONE (<35S)

0.0s35s Sub-9090s ESAT Budget150s Classroom

Question 1 of 1ARCH-C-02 (6-Card Lesson)Format: Standard 5-Option

Right-click any option to cross out • Keys A–E + Enter

Question 1 of 1 • 90s Target

Ready to Start Question 1?

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-02•Standard 5-Option

or press Enter to start