Formulae And Relative Masses

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Notes

Formulae

Elements are represented by **chemical symbols** from the Periodic Table.
Seven elements exist as **diatomic molecules** (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂).
A **molecular formula** shows the type and number of atoms in a molecule (e.g., H₂O, CH₄).
Ionic compounds are formed from metals and non-metals; the formula must have **no overall charge**.
Use the **swap-and-drop method** to determine the formula when ions have different charges (e.g., Cu²⁺ and Cl⁻ → CuCl₂).
Common polyatomic ions: **carbonate** CO₃²⁻, **sulfate** SO₄²⁻, **hydroxide** OH⁻, **nitrate** NO₃⁻, **ammonium** NH₄⁺.

Writing Equations

**Word equations** show reactants → products using full chemical names.
**Symbol equations** use chemical formulae and must be **balanced** to satisfy conservation of mass.
Include **state symbols**: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Balance by adjusting coefficients in front of formulae; never change subscripts.
**Ionic equations** show only the species that change; spectator ions are removed.
Example: 2Al(s) + 3Cl₂(g) → 2AlCl₃(s) is a balanced symbol equation.

Relative Atomic Mass (Aᵣ)

**Relative atomic mass (Aᵣ)** compares the mass of an atom to $\frac{1}{12}$ the mass of carbon-12.
Aᵣ values are found on the Periodic Table $(e.g., H=1, C=12, O=16)$ .
Carbon-12 is the standard with a fixed mass of 12 units.

Relative Formula Mass (Mᵣ)

**Relative formula mass (Mᵣ)** is the sum of the Aᵣ of all atoms in a formula.
For molecules, it is called **relative molecular mass**.
Calculate Mᵣ by multiplying each Aᵣ by the number of atoms and adding them (e.g., H₂O: $2\times 1 + 16 = 18)$ .
Examples: NaCl $= 58.5$ , Ca(OH)₂ $= 74$ , (NH₄)₂SO₄ $= 132$ .

Reacting Masses

**Conservation of mass**: total mass of reactants = total mass of products.
Use balanced equations and Mᵣ to calculate masses of reactants and products.
Example: 2Ca + O₂ → 2CaO: 80 g Ca reacts with 32 g O₂ to give 112 g CaO.
Mass ratios are fixed; doubling the reactant mass doubles the product mass.

Bohr model of a carbon atom showing 6 protons, 6 neutrons, and electron configuration 2,4.

Practice questions

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1.Which of the following elements exists as a diatomic molecule?
Easy
- AHelium
- BChlorine
- CCarbon
- DSulfur
2.What is the molecular formula of water?
Easy
3.Calculate the relative formula mass of calcium hydroxide, Ca(OH)2. $(Ar: Ca=40, O=16, H=1)$
Medium
- A81
- B67
- C74
- D88
4.Complete the sentence.
Medium
In a balanced chemical equation, the total mass of reactants equals the total mass of products due to the law $of ____$ .
5.The relative atomic mass of carbon is 12.
Easy
True or false?
6.Match each ion to its formula.
Medium
- Hydroxide
- Sulfate
- Ammonium
- OH-
- $SO4^{2}-$
- NH4+
7.Arrange the following steps for writing an ionic equation in the correct order.
Hard
- Replace ionic compounds with component ions
- Write the full balanced symbol equation
- Remove spectator ions
8.Calculate the mass of carbon dioxide produced when 32 g of methane (CH4) burns completely in excess oxygen. $(Ar: H=1, C=12, O=16) CH4 + 2O2 \to CO2 + 2H2O$
Hard
- A97
- B106
- C88
- D79

Slides

Molecular Formulae

A molecular formula shows the types and numbers of atoms in a molecule.
For example, H₂O means 2 hydrogen atoms and 1 oxygen atom.
Seven elements exist as diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

Which of the following is the molecular formula of ammonia?

NH₃
NH₄
N₂H₄
NH₂

Deducing Formulae of Ionic Compounds

Ionic compounds have no overall charge; the total positive charge equals total negative charge.
Use the swap-and-drop method: swap the charges of the ions and drop them as subscripts.
For example, Cu²⁺ and Cl⁻ give CuCl₂.

The formula of iron(III) oxide $is ____$ .

Writing Word Equations

Word equations show reactants on the left and products on the right, separated by an arrow.
The arrow means 'to form' or 'produces'.
Example: sodium hydroxide + hydrochloric acid → sodium chloride + water.

Which of these are correct word equations?

Carbon + oxygen → carbon dioxide
Ammonia + nitric acid → ammonium nitrate
Iron + copper(II) sulfate → iron(II) sulfate + copper
Water + carbon dioxide → glucose + oxygen

Balancing Symbol Equations

A balanced equation has the same number of each atom on both sides.
Use coefficients (numbers in front) to balance; never change subscripts.
Include state symbols: (s), (l), (g), (aq).

What is the balanced equation for the reaction of magnesium with oxygen?

Mg + O₂ → MgO
2Mg + O₂ → 2MgO
$Mg + O$ → MgO
Mg₂ + O₂ → 2MgO

Relative Atomic Mass (Ar)

Relative atomic mass (Ar) compares the mass of an atom to $\frac{1}{12}$ of a carbon-12 atom.
Ar values are found on the Periodic Table $(e.g., H=1, C=12, O=16)$ .
Carbon-12 is the standard with a mass of exactly 12.

The relative atomic mass of magnesium $is ____$ .

Relative Formula Mass (Mr)

Mr is the sum of the Ar of all atoms in a formula.
For H₂O: $(2\times 1) + 16 = 18$ .
For ionic compounds, it's called relative formula mass.

What is the Mr of calcium carbonate, CaCO₃? $(Ar: Ca=40, C=12, O=16)$

Reacting Masses

Mass is conserved in reactions: total mass of reactants = total mass of products.
Use balanced equations to find mass ratios.
Example: 2Ca + O₂ → 2CaO; 80 g Ca reacts with 32 g O₂ to give 112 g CaO.

In the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, 16 g of CH₄ produces $____ g$ of CO₂.

Deducing Symbol Equations (Extended)

You may need to deduce formulae and then balance.
Example: Al + Cl₂ → AlCl₃. Balanced: 2Al + 3Cl₂ → 2AlCl₃.
Remember diatomic elements: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

What is the balanced equation for aluminium reacting with oxygen?

Al + O₂ → Al₂O₃
4Al + 3O₂ → 2Al₂O₃
2Al + O₂ → Al₂O₃
$Al + O$ → AlO

Ionic Equations (Extended)

Ionic equations show only the ions that change.
Write full balanced equation, then split into ions, then cancel spectator ions.
Example: 2I⁻ + Cl₂ → 2Cl⁻ + I₂.

Which of these are spectator ions in the reaction 2KI(aq) + Cl₂(aq) → 2KCl(aq) + I₂(aq)?

K⁺
I⁻
Cl⁻
Cl₂

Flashcards

What is the relative atomic mass (Ar) of an element?

The relative atomic mass is the average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.

How is relative molecular mass (Mr) calculated?

Mr is the sum of the relative atomic masses of all atoms in a molecule or formula unit.

Calculate the relative formula mass of water (H2O). $(Ar: H=1, O=16)$

$Mr = (2\times 1) + 16 = 18$ .

Calculate the relative formula mass of magnesium nitrate, Mg(NO3)2. $(Ar: Mg=24, N=14, O=16)$

$Mr = 24 + 2\times (14+3\times 16) = 24 + 2\times 62 = 148$ .

What is the law of conservation of mass?

Mass cannot be created or destroyed; in a chemical reaction, the total mass of reactants equals the total mass of products.

If 80 g of calcium reacts with 32 g of oxygen to form calcium oxide, what mass of calcium oxide is produced?

$112 g (80 + 32 = 112)$ .

What is the molecular formula of a compound?

It shows the type and number of atoms of each element in a molecule.

Name the seven diatomic elements.

Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2), Iodine (I2).

What is the formula of iron(III) chloride?

FeCl3 (Fe3+ and Cl- combine in a 1:3 ratio).

What is the formula of copper(II) chloride?

CuCl2 (Cu2+ and Cl- combine in a 1:2 ratio).

What is the formula of ammonium sulfate?

(NH4)2SO4 (two NH4+ ions combine with one SO42- ion).

Write the balanced symbol equation for the reaction of aluminium with chlorine to form aluminium chloride.

$2Al (s) + 3Cl2 (g)$ → 2AlCl3 (s).

Write the balanced symbol equation for the reaction of magnesium oxide with nitric acid.

MgO $(s) + 2HNO3 (aq)$ → $Mg(NO3)2 (aq) + H2O (l)$ .

What are state symbols?

They indicate the physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

Write the ionic equation for the reaction of chlorine with aqueous potassium iodide.

$2I- (aq) + Cl2 (aq)$ → $2Cl- (aq) + I2 (aq)$ .

Calculate the mass of carbon dioxide produced when 32 g of methane (CH4) burns completely in excess oxygen. $(Ar: H=1, C=12, O=16)$

From $CH4 + 2O2$ → $CO2 + 2H2O, 16 g CH4$ produces 44 g CO2, so 32 g produces 88 g CO2.

What is the difference between relative atomic mass and relative molecular mass?

Relative atomic mass (Ar) applies to atoms; relative molecular mass (Mr) applies to molecules or formula units.

What is the formula of calcium hydroxide?

Ca(OH)2 (Ca2+ and OH- combine in a 1:2 ratio).

What is the formula of potassium carbonate?

K2CO3 (two K+ ions combine with one CO32- ion).

Balance the equation: $_{Al} + _{CuO}$ → $_{Al2O3} + _{Cu}$

$2Al + 3CuO$ → $Al2O3 + 3Cu$ .

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