FormulaeCambridge IGCSE Chemistry: Revision notes

Molecular formula shows the actual number and type of atoms in one molecule of a compound. Empirical formula shows the simplest whole number ratio of different atoms or ions in a compound.

For example:

• Hydrogen peroxide has a molecular formula of H₂O₂ but an empirical formula of HO
• Glucose has a molecular formula of C₆H₁₂O₆ but an empirical formula of CH₂O

The molecular formula is always a multiple of the empirical formula. To find the molecular formula, you need to know the relative molecular mass (Mr) of the compound.

When given the relative numbers of atoms or ions in a model or diagram:

1. Count the atoms or ions of each element present
2. Write down the ratio of atoms/ions (e.g. 2:1:3)
3. Simplify the ratio to the smallest whole numbers by dividing by the highest common factor
4. Write the formula using the simplified ratio as subscripts

For ionic compounds, deduce the formula from the charges on the ions:

• The positive charges must equal the negative charges
• Work out how many ions of each type are needed to balance the charges
• For example: magnesium oxide has Mg²⁺ and O²⁻ ions, so one of each is needed → MgO
• Calcium chloride has Ca²⁺ and Cl⁻ ions, so two Cl⁻ ions are needed for one Ca²⁺ → CaCl₂

Word equations show the names of reactants and products separated by an arrow:

Reactant + Reactant → Product + Product

Symbol equations show the chemical formulae of reactants and products:

Chemical formula + Chemical formula → Chemical formula + Chemical formula

Symbol equations must be balanced so that the number of atoms of each element is the same on both sides.

State symbols must be included in symbol equations:

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous (dissolved in water)

Steps to write a balanced symbol equation:

1. Write the unbalanced equation using correct formulae
2. Count atoms of each element on both sides
3. Add numbers (coefficients) in front of formulae to balance
4. Check all elements are balanced
5. Add state symbols

Example: Magnesium + oxygen → magnesium oxide Unbalanced: Mg + O₂ → MgO Balanced: 2Mg + O₂ → 2MgO

Ionic equations show only the ions and atoms that actually participate in the chemical reaction, omitting spectator ions (ions that do not change during the reaction).

Steps to write an ionic equation:

1. Write the full balanced symbol equation with state symbols
2. Write the full ionic equation by showing all ionic compounds in their ionised form (only for compounds that are soluble and ionic)
3. Identify spectator ions (ions that appear unchanged on both sides)
4. Cancel spectator ions from both sides
5. Write the net ionic equation showing only the particles that react

Example: Full equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) Full ionic: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l) Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l)

Na⁺ and Cl⁻ are spectator ions as they don't change.

You can work out a symbol equation if given:

• The names of reactants and products
• The state symbols
• Information about the reaction (e.g. type of reaction, or partial equations)
• Data such as masses, volumes, or molar amounts

Key steps:

1. Identify the reactants (starting materials) and products (substances formed)
2. Write the formulae of all substances (use your knowledge of common ions and compounds)
3. Write the unbalanced equation
4. Balance the equation by counting atoms of each element
5. Add state symbols based on the reaction conditions

Common reaction types and products:

• Combustion: fuel + oxygen → carbon dioxide + water (if contains C and H)
• Oxidation: element + oxygen → oxide
• Acid-base: acid + base → salt + water
• Displacement: more reactive element + compound → new compound + less reactive element
• Decomposition: compound → simpler substances

Example: Deduce the equation for the reaction between sodium carbonate solution and dilute hydrochloric acid: Reactants: Na₂CO₃(aq) and HCl(aq) Products: NaCl, CO₂, H₂O Unbalanced: Na₂CO₃(aq) + HCl(aq) → NaCl(aq) + CO₂(g) + H₂O(l) Balanced: Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + CO₂(g) + H₂O(l)