SPI Chemistry · Free notes

Mole Concept and Stoichiometry: SPI Chemistry

Mole Concept and Stoichiometry has 6 questions in the SPI bank, two from Practice Paper 1 and four from the LWS mocks, and none from the 2022 paper. Two are EASY, three MODERATE and one HARD. None has a number for its answer, though two need a short calculation to decide; all of them test whether you know what a formula, a molecule or a mole actually stands for. The wrong options are near misses (CaCl for CaCl₂, diatomic argon, molecules counted where atoms are asked), so these pages put the rule beside every example.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

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Subtopic notes

Formula & revision sheet

4 formulas · 1 reference tables · 10 gotchas across all subtopics — the exam-eve cheat-sheet

Laws of Combination and Chemical Formulae

Formulas (2)

The laws of chemical combination · Mass fraction of an element $\text{fraction of element} = \frac{\text{mass of element}}{\text{mass of compound}}$
Valency and writing a correct formula · Balanced formula $\text{A}^{x}\,\text{B}^{y} \;\Rightarrow\; \text{A}_y\text{B}_x \quad (\text{total valency of A} = \text{total valency of B})$

Reference tables (1)

Atomicity: how many atoms in one molecule5 rows

Atomicity	Name	Examples
1	Monoatomic	Helium, neon, argon (noble gases); metals such as sodium
2	Diatomic	H₂, O₂, N₂, Cl₂, F₂, Br₂, I₂
3	Triatomic	O₃ (ozone)
4	Tetratomic	P₄ (phosphorus)
8	Octatomic (polyatomic)	S₈ (sulphur)

Oxygen is diatomic as O₂ but triatomic as ozone, O₃.

Watch out for (6)

Choosing multiple proportions for one compound
→ The laws of chemical combination
Comparing masses instead of fractions
→ The laws of chemical combination
Treating every gas as diatomic
→ Atomicity: how many atoms in one molecule
Forgetting ozone
→ Atomicity: how many atoms in one molecule
Writing one of each without checking valency
→ Valency and writing a correct formula
Leaving out the brackets
→ Valency and writing a correct formula

The Mole and Avogadro's Number

Formulas (2)

Moles, Avogadro's number and counting atoms · Moles and particles $n = \frac{m}{M} \qquad N = n \times N_A \qquad N_A = 6.022 \times 10^{23}$
Charge needed to deposit one mole of a metal · Charge to deposit one mole of metal $Q = n \times 96500\ \text{C} \quad \text{for ions of charge } n+$

Watch out for (4)

Counting molecules when atoms are asked
→ Moles, Avogadro's number and counting atoms
Dividing by Avogadro's number
→ Moles, Avogadro's number and counting atoms
Thinking every metal needs one faraday per mole
→ Charge needed to deposit one mole of a metal
Grouping metals by their mass instead of their charge
→ Charge needed to deposit one mole of a metal

Question weightage by concept

5 concepts · 6 questions — where the marks actually sit, so you know what to drill first

Laws of Combination and Chemical Formulae3 questions · 50%

Concept	Qs	Share
The laws of chemical combination	1	17%
Atomicity: how many atoms in one molecule	1	17%
Valency and writing a correct formula	1	17%

The Mole and Avogadro's Number3 questions · 50%

Concept	Qs	Share
Moles, Avogadro's number and counting atoms	2	33%
Charge needed to deposit one mole of a metal	1	17%

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