Study Notes
In chemistry, formulae are used to represent ionic compounds, balance chemical equations, and determine empirical and molecular formulas. Water crystallisation involves understanding the role of water in crystal structures.
- Ionic Compounds — compounds made up of positive and negative ions held together by ionic bonds. Example: Sodium chloride (NaCl) is formed from Na+ and Cl- ions.
- Balancing Equations — ensuring the number of atoms of each element is equal on both sides of a chemical equation. Example: 2H2 + O2 -> 2H2O balances hydrogen and oxygen atoms.
- Empirical Formula — the simplest whole number ratio of elements in a compound. Example: The empirical formula of glucose is CH2O.
- Molecular Formula — the actual number of atoms of each element in a molecule. Example: The molecular formula of glucose is C6H12O6.
- Water Crystallisation — refers to water molecules that are part of the crystal structure of a hydrated compound. Example: Copper(II) sulfate pentahydrate (CuSO4·5H2O) contains water of crystallisation.
Exam Tips
Key Definitions to Remember
- Ionic compounds are made of cations and anions.
- Empirical formula is the simplest ratio of elements.
- Molecular formula shows the actual number of atoms.
- Water of crystallisation is water in a crystal structure.
Common Confusions
- Confusing empirical and molecular formulas.
- Forgetting to balance charges in ionic compounds.
Typical Exam Questions
- What is the empirical formula of a compound with 40% carbon, 6.7% hydrogen, and 53.3% oxygen? CH2O
- How do you balance the equation for the combustion of methane? CH4 + 2O2 -> CO2 + 2H2O
- What is the molecular formula of a compound with an empirical formula of CH2 and a molar mass of 56 g/mol? C4H8
What Examiners Usually Test
- Ability to write and balance chemical equations.
- Understanding of the difference between empirical and molecular formulas.
- Knowledge of ionic compound formation and water crystallisation.