Chemical Calculations and Moles IGCSE Chemistry: Complete Guide

🧮 Understanding Chemical Calculations in IGCSE Chemistry

Chemical calculations are essential for quantitative chemistry. This guide covers moles, molar mass, and stoichiometry with step-by-step methods.

🧠 Memory Tips

Mole Calculations: “Mass Moles Molecules”

• Mass = Moles × Molar mass
• Moles = Mass/Molar mass
• Molecules = Moles × Avogadro’s number

Avogadro’s Number: “6.02 × 10²³”

• Number of particles in one mole
• Remember as “Six-oh-two times ten to twenty-three”

Stoichiometry Steps: “Balance Convert Calculate”

• Balance the equation
• Convert to moles
• Calculate using ratios

📚 Key Concepts

The Mole

• Definition: Amount of substance containing 6.02 × 10²³ particles
• Avogadro’s number: 6.02 × 10²³
• Symbol: mol
• Applications: Counting atoms, molecules, ions

Molar Mass

• Definition: Mass of one mole of substance
• Units: g/mol
• Calculation: Sum of atomic masses
• Example: H₂O = (2 × 1) + (1 × 16) = 18 g/mol

Key Formulas

• n = m/M (moles = mass/molar mass)
• m = n × M (mass = moles × molar mass)
• N = n × Nₐ (particles = moles × Avogadro’s number)

Stoichiometry

• Definition: Quantitative relationships in chemical reactions
• Mole ratios: From balanced equations
• Applications: Predict products, calculate yields

🎯 IGCSE Exam Focus

Common Questions

1. Calculate number of moles (3 marks)
2. Find molar mass of compounds (4 marks)
3. Stoichiometry calculations (6 marks)
4. Percentage composition (5 marks)

Problem-Solving Steps

1. Write balanced equation
2. Identify given and required
3. Convert to moles
4. Use mole ratios
5. Convert back to required units

Common Calculations

• Moles from mass: n = m/M
• Mass from moles: m = n × M
• Particles from moles: N = n × Nₐ
• Percentage composition: (Element mass/Total mass) × 100%

🔢 Calculation Examples

Example 1: Finding Moles

Question: How many moles in 36g of water (H₂O)? Solution:

• Molar mass of H₂O = 18 g/mol
• n = m/M = 36/18 = 2 mol

Example 2: Stoichiometry

Question: How much CO₂ from 12g of carbon? Equation: C + O₂ → CO₂ Solution:

• Moles of C = 12/12 = 1 mol
• Mole ratio C:CO₂ = 1:1
• Moles of CO₂ = 1 mol
• Mass of CO₂ = 1 × 44 = 44g

Percentage Composition

Example: Find % of oxygen in H₂O Solution:

• Mass of O in H₂O = 16g
• Molar mass of H₂O = 18g
• % O = (16/18) × 100% = 88.9%

📊 Gas Calculations

Molar Volume

• At STP: 22.4 L/mol
• At room conditions: 24 L/mol
• Formula: V = n × Vm

Gas Stoichiometry

• Volume ratios: Same as mole ratios
• Example: 2H₂ + O₂ → 2H₂O
• Ratio: 2:1:2 (volumes)

🧪 Practical Applications

Empirical Formulas

• Definition: Simplest whole number ratio
• Method: Convert masses to moles, find ratios
• Example: CH₂O from glucose C₆H₁₂O₆

Molecular Formulas

• Definition: Actual number of atoms
• Relationship: Molecular = n × Empirical
• Need: Molecular mass

Yield Calculations

• Theoretical yield: Maximum possible
• Actual yield: What is obtained
• Percentage yield: (Actual/Theoretical) × 100%

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