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

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-C-02: Why the 140s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~140s (Blows 90s Clock)

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).

The Fast ShortcutTarget: 35s (Saves 105s)

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).

Where Students Lose Time: 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.

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