⚖️ Stoichiometry
Definition
Stoichiometry is derived from Greek words “stoicheion” meaning element and “metron” meaning measure. It is the branch of chemistry that studies the relationship between the amounts of reactants and products in a balanced chemical equation.
Laws Governing Stoichiometry
Law of Conservation of Mass: Matter (mass) can neither be created nor destroyed. The total mass of reactants equals the total mass of products.
Law of Definite Proportions: A pure compound always contains the same elements combined in the same ratio by mass.
Key Concept: Stoichiometric calculations obey both the law of conservation of mass and the law of definite proportions.
🧪 Concept of Mole
Definition of Mole
The mole is the amount of a substance which contains as many elementary entities as there are atoms in 0.012 kg (12 g) of carbon-12.
Avogadro’s Number: 6.02 × 10²³ entities per mole. Represented by Nₐ.
Examples of 1 Mole
1 mole of ¹²C contains 6.02 × 10²³ atoms of ¹²C
1 mole of H₂O contains 6.02 × 10²³ molecules of H₂O
1 mole of NaCl contains 6.02 × 10²³ formula units of NaCl
1 mole of Na⁺ contains 6.02 × 10²³ ions of Na⁺
Number of moles (n) = Mass (g) ÷ Molar Mass (g/mol)
📊 Molar Mass Calculations
Definition
The mass of one mole of a substance expressed in grams is called molar mass. Unit: g/mol.
Sample Calculations
Example 1: Molar mass of CCl₄ = 12.0 × 1 + 35.5 × 4 = 154.0 g/mol
Example 2: 1 mole of CO₂ molecules = 44.0 g
Example 3: 1 mole of CaO formula units = 56.1 g
Molar Mass = Sum of atomic masses of all atoms in the formula
⚗️ Molar Volume of Gases
Definition
The volume occupied by one mole of an ideal gas at STP (0°C, 1 atm) is called molar volume. Value: 22.4 dm³ at STP.
Avogadro’s Law
Equal volumes of all ideal gases at the same temperature and pressure contain equal numbers of molecules.
Examples at STP
22.4 dm³ of CO₂ = 44.0 g = 6.02 × 10²³ molecules = 1 mole
22.4 dm³ of H₂ = 2.0 g = 6.02 × 10²³ molecules = 1 mole
22.4 dm³ of NH₃ = 17.0 g = 6.02 × 10²³ molecules = 1 mole
Volume of gas at STP = Number of moles × 22.4 dm³
🔗 Stoichiometric Relationships
Types of Relationships
1. Mole-Mole Relationship: Using mole ratios from balanced equations
2. Mass-Mass Relationship: Converting between masses of reactants/products
3. Volume-Volume Relationship: For gases at same T & P
4. Mole-Mass Relationship: Converting between moles and mass
5. Mole-Volume Relationship: For gases at STP
6. Mass-Volume Relationship: Between mass and gas volume
Assumptions
All reactants completely converted to products
No side reactions occur
Laws of conservation of mass and definite proportions are obeyed
🚀 Study Strategies
1
Master Mole Conversions
Practice converting between mass, moles, and number of particles using Avogadro’s number and molar mass.
2
Balance Equations First
Always start stoichiometry problems by balancing the chemical equation to get correct mole ratios.
3
Use Dimensional Analysis
Set up conversion factors to cancel units systematically and avoid calculation errors.