Chapter 2: Atoms, Molecules and Stoichiometry

Chapter 2: Atoms, Molecules and Stoichiometry

The Mole Concept

- Mole: A quantity representing 6.022 x 10^23 particles (Avogadro’s number).

- Used to count atoms, ions, and molecules in chemical reactions.

- Molar mass (g/mol): Mass of one mole of a substance, numerically equal to its relative atomic/molecular mass.

Empirical and Molecular Formulae

- Empirical formula: Simplest whole-number ratio of atoms.

- Molecular formula: Actual number of atoms in a molecule.

- To find empirical formula:

  1. Convert mass to moles.

  2. Divide by the smallest mole value.

  3. Multiply to get whole numbers if needed.

Chemical Equations and Stoichiometry

- Balanced equations show mole ratios of reactants and products.

- Stoichiometry: Using mole ratios to calculate quantities in reactions.

- Steps:

  1. Write a balanced equation.

  2. Convert known mass to moles.

  3. Use the mole ratio to find the unknown.

  4. Convert moles back to mass if needed.

Limiting Reagent

- The reactant that runs out first, limiting the amount of product formed.

- Identify by comparing mole ratios from the balanced equation.

Yield and Purity

- Percentage yield = (actual yield / theoretical yield) × 100

- Percentage purity = (mass of pure substance / total mass) × 100

Concentration of Solutions

- Concentration (mol/dm³) = moles of solute / volume of solution (dm³)

- To prepare solutions:

  - Use molar mass to calculate the required mass.

  - Dissolve and dilute to the desired volume.

Ideal Gas Equation

- pV = nRT

  - p: pressure (Pa)

  - V: volume (m³)

  - n: moles

  - R: gas constant (8.31 J/mol·K)

  - T: temperature (K)

- Use to calculate volume, pressure, or moles of gases.