SPI Chemistry · Mole Concept and Stoichiometry

Laws of Combination and Chemical Formulae

Elements combine in fixed proportions by mass, each element exists as molecules of a set number of atoms, and a correct formula balances the valencies of its parts.

All Mole Concept and Stoichiometry notes

Why this matters

Three questions in the bank, one EASY and two MODERATE: which law two copper oxide experiments show, the atomicity of chlorine and argon, and which of four formulae is written correctly. Each is a definition or a valency rule, and each wrong option is a near miss, such as CaCl for CaCl₂.

Concept 1 of 3: The laws of chemical combination

Water from a river and water made in a laboratory are the same compound, so they must be built the same way. Weigh the hydrogen and oxygen in any sample and the share of each never changes. That steadiness is what the laws of combination describe.

Definition

Law of conservation of mass: in a chemical reaction, mass is neither created nor destroyed. Total mass of reactants $=$ total mass of products.
Law of constant proportions: a pure compound always contains the same elements combined in the same proportion by mass, however it is made. Check it by working out the fraction of one element in each sample.
Law of multiple proportions: when two elements form more than one compound, the masses of one that combine with a fixed mass of the other are in a simple whole-number ratio (carbon gives CO and CO₂: 16 g and 32 g of oxygen with 12 g of carbon, ratio 1 : 2).
So the same fraction in two samples of one compound means constant proportions; two different compounds of the same elements point to multiple proportions.

Mass fraction of an element

\text{fraction of element} = \frac{\text{mass of element}}{\text{mass of compound}}

Worked example

One sample of water of mass 18 g contains 2 g of hydrogen. Another sample of mass 45 g contains 5 g of hydrogen. Which law do these show?

Practice this conceptself-check · 3 quick reps

The same idea in a practice question:

Practice questionModerate

Example 1 · Mole Concept and Stoichiometry · Laws of Combination and Chemical Formulae

In an experiment, 4.90 g of copper oxide was obtained from 3.92 g of copper. In another experiment, 4.55 g of copper oxide gave, on reduction, 3.64 gram of copper. Which of the law of chemical combinations is illustrated by these experiments?

Choosing multiple proportions for one compound

Multiple proportions needs two different compounds of the same two elements. Two samples of the same oxide with the same fraction of metal show constant proportions.

Comparing masses instead of fractions

Two samples of a compound usually have different total masses. Divide the mass of one element by the mass of the sample before comparing.

Concept 2 of 3: Atomicity: how many atoms in one molecule

Some elements are happy as lone atoms; others pair up or form small clusters. The number of atoms joined in one molecule of the element is its atomicity, and a handful of them are worth knowing by heart.

Definition

Atomicity is the number of atoms in one molecule of an element.
Noble gases (helium, neon, argon) exist as single atoms: monoatomic.
Hydrogen, oxygen, nitrogen and the halogens (fluorine, chlorine, bromine, iodine) form pairs: diatomic.
Ozone $\text{O}_3$ is triatomic, phosphorus $\text{P}_4$ tetratomic and sulphur $\text{S}_8$ has eight atoms.

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.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionEasy

Example 2 · Mole Concept and Stoichiometry · Laws of Combination and Chemical Formulae

Atomicity of Chlorine and Argon is:

Treating every gas as diatomic

Hydrogen, oxygen, nitrogen and chlorine are diatomic, but the noble gases (helium, neon, argon) are monoatomic.

Forgetting ozone

Oxygen has two forms: ordinary oxygen O₂ (atomicity 2) and ozone O₃ (atomicity 3). Read which one the question names.

Concept 3 of 3: Valency and writing a correct formula

Think of valency as the number of hands an atom or group has to hold others. A formula is correct when every hand is held: the total valency on the positive side equals the total on the negative side.

Definition

Valency is the combining capacity of an atom or a group of atoms (a radical).
Valency 1: H, Na, K, Ag, NH₄ (ammonium); Cl, OH (hydroxide), NO₃ (nitrate).
Valency 2: Mg, Ca, Zn, Cu; O, S (sulphide), SO₄ (sulphate), CO₃ (carbonate).
Valency 3: Al, Fe (in iron(III) compounds), Bi; PO₄ (phosphate).
Criss-cross: write the symbols, swap the valencies to become the subscripts, and cancel any common factor. Put a group in brackets when it needs a subscript more than 1.
Equal valencies cancel to 1 : 1, so no subscripts are needed.

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})

Worked example

Write the formula of aluminium sulphate.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionModerate

Example 3 · Mole Concept and Stoichiometry · Laws of Combination and Chemical Formulae

Which of the following represents a correct chemical formula? Name it.

Writing one of each without checking valency

Sodium (valency 1) and sulphur in a sulphide (valency 2) do not pair one to one: sodium sulphide is

\text{Na}_2\text{S}

. Always balance the totals.

Leaving out the brackets

Two sulphate groups are written

(\text{SO}_4)_2

, not

\text{SO}_{42}

. The bracket shows the whole group is doubled.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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

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