IGCSE Chemistry: Stoichiometry (Part 1)

Introduction

• Channel: IGCSE Study Buddy
• Topic: Stoichiometry (Part 1)
• Purpose: Revise chemistry topics from the Cambridge IGCSE syllabus

Chemical Formulas

• Definition: A representation of elements and their proportions in a compound
• Example: NaCl (sodium chloride) - 1 sodium atom and 1 chlorine atom
• Element Symbols: Found in the periodic table
  • Examples:
    • H for Hydrogen
    • O for Oxygen
    • Na for Sodium

Writing Chemical Formulas

• First letter of a two-letter symbol is capitalized, second is lowercase (e.g., Mg for Magnesium)
• Numbers indicate the number of atoms (e.g., O2 for two oxygen atoms)
• Molecular formula: Number and type of atoms in a molecule (e.g., Methane as CH4)

Empirical vs Molecular Formulas

• Molecular Formula: Shows actual number of atoms in a compound
• Empirical Formula: Simplest whole number ratio of atoms in a compound
  • Example:
    • Ethanol: Molecular formula C2H5OH, empirical formula C2H6
• Conversion: Divide subscripts by the lowest common denominator

Valency

• Definition: The ability of an atom to bond with others
• Determination: Based on group number in the periodic table
  • Groups 1-3 lose electrons (metals)
  • Groups 5-7 gain electrons (non-metals)
  • Group 4 can gain or lose electrons

Determining Compound Formulas

• Aluminium Oxide Example:
  • Aluminium (Al) valency: 3
  • Oxygen (O) valency: 2
  • Formula: Al2O3 by crisscrossing valencies

Ionic Compounds

• Ionic Bonds:
  • Formed by transfer of electrons
  • Charge balance: Positive and negative charges cancel out
• Example: Potassium Chloride (KCl)
  • Potassium (K+) and Chloride (Cl-) ions balance in a 1:1 ratio

Practical Examples

• Boron Oxide (B2O3):
  • Boron (B) charge: +3
  • Oxygen (O) charge: -2
  • Ratio: 2 boron ions to 3 oxygen ions

Criss-Cross Method

• Used to Determine: Simplest ratio for neutral ionic compound
• Example: Calcium Nitrate
  • Calcium (+2) and Nitrate (-1)
  • Formula: Ca(NO3)2
• Example: Ammonium Carbonate
  • Formula considerations for compound ions

Conclusion

• Content: Part 1 of Stoichiometry completed
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