IGCSE Study Buddy: Stoichiometry (Part 1)

Introduction

• Welcome to IGCSE Study Buddy for revising chemistry topics from the Cambridge IGCSE syllabus.
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Key Concepts in Stoichiometry

Chemical Formulas

• Definition: Representation of elements and their proportions in a compound.
• Example: Table salt is represented as NaCl (1 sodium atom + 1 chlorine atom).
• Elements are represented with unique chemical symbols from the periodic table (e.g., H for hydrogen, O for oxygen, Na for sodium).
  • Note: For two-letter symbols, the first letter is capitalized, and the second is lowercase (e.g., Mg for magnesium).
  • Subscripts: Indicate the number of atoms (e.g., O₂ = 2 oxygen atoms).
  • Single Atoms: Do not write the number one (e.g., H instead of H₁).

Molecular and Empirical Formulas

• Molecular Formula: Indicates the actual number of atoms of each element in a molecule (e.g., CH₄ for methane).
• Empirical Formula: Simplest whole number ratio of atoms in a compound.
  • Example: Ethanol C₂H₅OH has a molecular formula of C₂H₅OH and an empirical formula of CH₃.
  • Finding Empirical Formulas: Divide subscripts by the lowest common denominator.

Valency

Definition

• Valency: Indicates how many bonds an atom can form by gaining, losing, or sharing electrons.
  • Example: Valency of 1 means losing or sharing one electron for stability.

Valency Table and Group Behavior

• Metals (Groups 1 to 3): Lose electrons.
• Non-metals (Groups 5 to 7): Gain electrons.
• Group 4: Can gain or lose electrons.

Example: Aluminium Oxide (Al₂O₃)

• Aluminium (Al) has a valency of 3, and oxygen (O) has a valency of 2.
• Crisscross Method: Combine valencies to determine formula: Al₂O₃.
• If valencies are equal, cancel them out.

Ionic Compounds

Charges of Ions

• Positive and negative charges must balance for a neutral compound.
• Common Ions: E.g., sodium ion (Na⁺), chloride ion (Cl⁻), copper ion (Cu²⁺).

Example: Potassium Chloride (KCl)

• Potassium loses one electron (K⁺); chlorine gains one electron (Cl⁻).
• Ratio is 1:1 for K⁺ and Cl⁻, resulting in KCl.

Example: Boron Oxide (B₂O₃)

• Boron has a charge of +3, and oxygen has a charge of -2.
• To balance charges: 2 boron ions (total +6) and 3 oxygen ions (total -6).

Criss-Cross Method for Ionic Compounds

• Method: Crisscross the charges to find the simplest ratio.
  • If charges are the same, it’s 1:1, no need to write '1'.
  • For ions with different charges, enclose multiple ions in brackets.

Examples

• Calcium Nitrate: Ca(NO₃)₂ (calcium has +2, nitrate has -1).
• Ammonium Carbonate: (NH₄)₂CO₃ (two ammonium ions).

Conclusion

• This concludes Part 1 of Stoichiometry.
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