Formulae | IGCSE Chemistry Revision Notes

1. Overview

Chemical formulae and equations are the universal language of chemistry. This topic focuses on how we represent individual atoms, molecules, and ions, and how we use these symbols to describe chemical reactions precisely through balanced equations.

Key Definitions

Molecular Formula: The actual number and type of different atoms present in one molecule of a compound.
Empirical Formula: The simplest whole-number ratio of the different atoms or ions present in a compound.
Word Equation: A chemical reaction described using the full names of the reactants and products.
Symbol Equation: A chemical reaction represented using chemical formulae, showing the ratio of substances and their physical states.
State Symbols: Notations used in equations to show the physical state of a substance: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.

Core Content

Common Formulae to Know

You are expected to recognize and use the formulae for common elements and compounds:

Elements: $H_2$, $O_2$, $N_2$, $Cl_2$, $Br_2$, $I_2$ (Diatomic molecules)
Compounds: $H_2O$ (Water), $CO_2$ (Carbon dioxide), $CO$ (Carbon monoxide), $CH_4$ (Methane), $NH_3$ (Ammonia), $HCl$ (Hydrochloric acid), $H_2SO_4$ (Sulfuric acid).

Deducing Formulae from Models

If given a diagram of a molecule, count the number of each type of atom to determine the formula.

Example: A molecule containing 1 Carbon atom and 4 Hydrogen atoms has the formula $CH_4$.
📊A central black sphere (Carbon) bonded to four smaller white spheres (Hydrogen) arranged in a tetrahedron shape.

Constructing Equations

Chemical equations show the transformation of reactants (left side) into products (right side).

Step 1: Word Equation Methane + Oxygen → Carbon dioxide + Water

Step 2: Symbol Equation (Balanced with State Symbols) $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$

State Symbols

(s): Solids (e.g., metals, precipitates like $AgCl$)
(l): Pure liquids (e.g., $H_2O$, $Hg$)
(g): Gases (e.g., $H_2$, $CO_2$)
(aq): Aqueous (substances dissolved in water, e.g., $NaCl$ solution)

Extended Content (Extended Curriculum Only)

Empirical Formula

The empirical formula is the simplest ratio.

Example: The molecular formula of Glucose is $C_6H_{12}O_6$. The ratio 6:12:6 can be simplified to 1:2:1. Therefore, the empirical formula is $CH_2O$.

Deducing Ionic Formulae

To write the formula of an ionic compound, the total positive charge must equal the total negative charge (net charge = 0).

Example 1: Magnesium ($Mg^{2+}$) and Chloride ($Cl^-$). To balance charges, we need two $Cl^-$ ions for every one $Mg^{2+}$ ion. Formula: $MgCl_2(s)$.
Example 2: Aluminium ($Al^{3+}$) and Oxide ($O^{2-}$). The lowest common multiple of 3 and 2 is 6. We need two $Al^{3+}$ and three $O^{2-}$. Formula: $Al_2O_3(s)$.

Ionic Equations

Ionic equations focus only on the particles that take part in the reaction, removing "spectator ions" (ions that remain unchanged in solution).

Full Equation: $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$

Ionic Equation: $Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$

Deducing Equations from Information

You may be asked to turn a description into a symbol equation.

Task: "Solid copper(II) carbonate reacts with dilute sulfuric acid to form aqueous copper(II) sulfate, carbon dioxide gas, and liquid water."
Deduction: $CuCO_3(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + CO_2(g) + H_2O(l)$

Key Equations

While "Formulae" is a conceptual topic, the balancing of equations follows the Law of Conservation of Mass: $$\text{Total Mass of Reactants} = \text{Total Mass of Products}$$

Example Calculation Table:

Component	Reactant Side ($CH_4 + 2O_2$)	Product Side ($CO_2 + 2H_2O$)
Carbon (C)	1	1
Hydrogen (H)	4	4
Oxygen (O)	4	4

Common Mistakes to Avoid

❌ Wrong: Writing $H2O$ or H2O.
✓ Right: Using subscripts: $H_2O$.
❌ Wrong: Changing the formula to balance an equation (e.g., changing $H_2O$ to $H_2O_2$).
✓ Right: Only change the coefficients (the numbers in front, e.g., $2H_2O$).
❌ Wrong: Using (aq) for pure water.
✓ Right: Water is a pure liquid (l); (aq) is only for substances dissolved in water.
❌ Wrong: Forgetting diatomic elements (writing $O$ instead of $O_2$).

Exam Tips

Command Word "Deduce": This means you don't need to memorize the answer; look at the charges or the diagram provided in the question to figure it out.
Command Word "State": Give a brief answer without explanation (e.g., "State the formula of ammonia" → $NH_3$).
Real-world Contexts: Be prepared for "Thermal" contexts (thermal decomposition of carbonates) and "Chemical" contexts (neutralization between acids and bases).
State Symbols: In Paper 4 (Extended), you often lose 1 mark per equation if state symbols are missing or incorrect. Always double-check them.
Calculations: Typical values like 98.0 ($H_2SO_4$) or 58.5 ($NaCl$) often appear in formula-based calculations. Practice calculating Relative Formula Mass ($M_r$) to assist with deducing formulae.