Formulae And Relative Masses

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Lesson 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⁺.

Swap-and-drop method for copper(II) chloride

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 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.

Key for chemical symbols on the Periodic Table

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: 2×1 + 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.

Slides

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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.
Easy
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.
Easy
- Hydroxide
- Sulfate
- Ammonium
- OH-
- SO4²-
- NH4+
7.Arrange the following steps for writing an ionic equation in the correct order.
Medium
- 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 → CO2 + 2H2O
Hard
- A97
- B106
- C88
- D79

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 = (2×1) + 16 = 18.

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

Mr = 24 + 2×(14+3×16) = 24 + 2×62 = 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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