The Mole and the Avogadro Constant

Concentration Measurement

• Concentration: Amount of solute in a solution's volume.
• Units:
  • g/dm³
  • mol/dm³
• Conversion between units:
  • Use formula triangle: Moles, Mass, and Relative Molecular Mass (M_r).

The Mole and Avogadro Constant

• Mole (mol): Unit for amount of substance.
• Avogadro Constant: Number of particles in 1 mole is 6.02 x 10²³.

Calculating Amount of Substance

• Formula: Amount of Substance (mol) = Mass (g) / Molar Mass (g/mol)
• Molar Mass vs. Relative Molecular Mass:
  • Molar Mass has units (g/mol); M_r is unitless.
  • Example: M_r of Calcium is 40, Molar Mass is 40g/mol.

Calculations

(a) Amount of Substance

• E.g. Sodium Sulfate (Na₂SO₄):
  • M_r = (23x2) + 32 + (16x4) = 142
  • Amount = 426g / 142 = 3 mol

(b) Mass

• E.g. Sodium (Na):
  • M_r = 23
  • Mass = 23 x 0.5 mol = 11.5g

(c) Molar Mass

• E.g. Aluminum Chloride (AlCl₃):
  • Molar Mass = 534g / 4 mol = 133.5 g/mol

(d) Relative Atomic/Molecular Mass

• E.g. CaCOH:
  • Mass / Moles = Molar Mass: 138g / 2 = 69 g/mol
  • M_r using A_r of elements: 40 + 12 + 16 + 1 = 69

(e) Number of Particles

• Avogadro's formula: Number of particles = Avogadro constant x Amount
• E.g. CO₂: 1.5 mol x 6.02x10²³ = 9.03x10²³ molecules
  • For atoms: Multiply molecules by number of atoms in a molecule.

Gas Calculations

• Molar Gas Volume: 24 dm³ at RTP (20°C, 1 atmosphere)
• Formula triangle based on conditions (cm³ or dm³).

Stoichiometry

Reacting Masses

• E.g. Magnesium and Oxygen:
  • 2Mg + O₂ → 2MgO
  • Mole calculation ties stoichiometric coefficients.

Limiting Reactants

• Identify by calculating moles and using ratios.

Volume Conversion

• 1 dm³ = 1000 cm³
• Conversion factor for calculations.

Gas Volumes

• E.g. Oxygen at RTP:
  • Volume = 0.75 mol x 24dm³

Solution Concentrations

• Conversion between g/dm³ and mol/dm³.
• Example calculations included.

Titration Calculations

• Use formula triangle.
• Conversion between cm³ and dm³ crucial.
• Example: HCl with NaOH.

Empirical and Molecular Formulae

• Empirical: Simplest ratio.
• Molecular: Actual atom numbers based on empirical formula.
• Example calculations provided.

Percentage Yield, Composition, and Purity

Percentage Yield

• Reasons for less than theoretical yield.
• Formula: (Actual yield / Theoretical yield) x 100
• Example: NaOH and HCl reaction.

Percentage Composition by Mass

• Formula: (Total A_r of element / M_r of compound) x 100
• Example: Magnesium in MgCO₃.

Percentage Purity

• Formula: (Mass of pure substance / Total mass) x 100
• Example: NaCl in solution.

These notes cover key concepts in stoichiometry, focusing on moles, Avogadro's constant, and calculations involving concentrations, gases, stoichiometry, yields, and purities.