Study Notes
This topic covers the basics of chemical formulae, equations, and calculations, including how to determine the composition of compounds and perform various chemical calculations.
- Simple Compounds — Represented using symbols of elements and the relative number of atoms. Example: NaCl is made up of 1 Na atom and 1 Cl atom.
- Ionic Compounds — Formed by balancing charges between positive and negative ions. Example: AlCl3 is formed by balancing Al3+ and Cl- ions.
- Relative Atomic Mass (Ar) — Average mass of atoms of an element compared to carbon-12. Example: Ar of carbon is 12.
- Relative Molecular Mass (Mr) — Sum of the relative atomic masses in a molecule. Example: Mr of CO2 is 44.
- Mole — Amount of substance measured in moles (mol). Example: 1 mol of any substance contains 6.02 x 10^23 particles.
- Avogadro’s Constant — Number of particles in a mole. Example: 6.02 x 10^23 per mole.
- Molar Gas Volume — Volume occupied by 1 mol of gas at RTP. Example: 24 dm3 at RTP.
- Limiting Reagent — Reactant that is completely consumed in a reaction. Example: In a reaction, the limiting reagent determines the amount of product formed.
- Empirical Formula — Simplest whole number ratio of elements in a compound. Example: CH2O for glucose.
- Theoretical Yield — Maximum amount of product expected from a reaction. Example: Calculated using balanced equations.
- Actual Yield — Amount of product actually obtained. Example: Often less than theoretical yield due to losses.
- Percentage Yield — Ratio of actual yield to theoretical yield expressed as a percentage. Example: (Actual Yield/Theoretical Yield) x 100.
- Purity — Measure of how pure a substance is. Example: Pure substances contain no impurities.
Exam Tips
Key Definitions to Remember
- Simple Compounds
- Ionic Compounds
- Relative Atomic Mass (Ar)
- Relative Molecular Mass (Mr)
- Mole and Avogadro’s Constant
- Molar Gas Volume
- Limiting Reagent
- Empirical Formula
- Theoretical and Actual Yield
- Percentage Yield
- Purity
Common Confusions
- Confusing empirical and molecular formulas
- Miscalculating molar ratios in reactions
- Overestimating percentage yield (cannot exceed 100%)
Typical Exam Questions
- How do you calculate the empirical formula of a compound? Use the mass of each element to find moles, then divide by the smallest number of moles.
- What is the percentage composition of oxygen in CO2? Calculate using the formula: (mass of oxygen/total molar mass of CO2) x 100.
- How do you determine the limiting reagent in a reaction? Compare the mole ratio of reactants to the balanced equation.
What Examiners Usually Test
- Ability to write and balance chemical equations
- Calculating moles and using Avogadro’s constant
- Determining empirical and molecular formulas
- Calculating yields and identifying limiting reagents