Stoichiometry - Part 1

Introduction

• Channel: IGCSE Study Buddy
• Topic: Stoichiometry (Part 1)
• Objective: Cover basic concepts of stoichiometry in chemistry.

Chemical Formulas

Understanding Formulas

• Definition: A formula represents elements and their proportions in a compound.
• Example: Table salt - NaCl (sodium chloride)
  • Elements in Formula: Na (sodium) and Cl (chlorine).
• Element Symbols: Found in the periodic table.
  • Single letter: Always capitalized (e.g., H for hydrogen).
  • Two letters: First is capital, second is lowercase (e.g., Mg for magnesium).

Indicating Number of Atoms

• Numbers in Formulas: Indicate quantity of atoms.
  • Example: O2 for two oxygen atoms.
  • Single atom: Number '1' is not written (e.g., H for hydrogen).

Types of Formulas

Molecular Formula

• Definition: Shows the number and type of different atoms in a molecule.
• Example: Methane - CH4 (1 carbon, 4 hydrogen).

Empirical Formula

• Definition: Simplest whole number ratio of atoms in a compound.
• Comparison:
  • Molecular Formula: Actual number of atoms.
  • Empirical Formula: Simplified ratio of atoms.
• Example: Carbon Dioxide (CO2) same for both molecular and empirical.
• Finding Empirical Formula: Divide subscripts by their lowest common denominator.

Valency and Ionic Compounds

Valency

• Definition: Number of electrons an atom gains, loses, or shares to stabilize electron configuration.
• Periodic Table Groups:
  • Groups 1-3 (Metals): Lose electrons.
  • Groups 5-7 (Non-metals): Gain electrons.
  • Group 4: Can lose or gain electrons.

Making Formulas Using Valency

• Crisscross Method: Swap valencies to determine a compound's formula.
• Example: Aluminium Oxide
  • Aluminium (Al) with valency 3, Oxygen (O) with valency 2.
  • Formula Al2O3 after crisscrossing valencies.

Ionic Compounds

Understanding Ionic Bonding

• Ion Charges:
  • Sodium ion: Na+ (plus 1 charge).
  • Chloride ion: Cl- (minus 1 charge).
• Neutral Compounds: Positive and negative charges balance out.

Examples

• Potassium Chloride (KCl):
  • Potassium (K+), Chloride (Cl-), 1:1 ratio.
• Boron Oxide (B2O3):
  • Boron (B) valency +3, Oxygen (O) valency -2.
  • Ratio 2 boron ions to 3 oxygen ions for neutrality.

Formula Deduction

• Criss-Cross Method: Cross-multiply charges of ions.
  • Example: Calcium Nitrate is Ca(NO3)2.
  • Enclose compound ions in brackets when multiple ions are needed.

Conclusion

• End of Part 1 of Topic 3 - Stoichiometry.
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