The Mole - Higher: Calculations in Chemistry (AQA GCSE Chemistry)

Introduction

• The mole is a unit for the amount of substance in chemistry.
• It is essential for performing calculations involving chemical quantities.
• The number of particles in a substance can be determined using the Avogadro constant.
• Mass of product depends on the mass of the limiting reactant.

Key Concepts

Mole Concept

• Mole (mol): The amount of substance that contains the same number of particles (atoms, molecules, ions) as there are in 12 grams of carbon-12.
• Avogadro Constant: 6.02 x 10^23 particles per mole.
• Used to measure chemical amounts.

Mass of One Mole

• Relative Atomic Mass (_A_r): The mean relative mass of atoms of different isotopes in an element.
• Relative Formula Mass (_M_r): The sum of the relative atomic masses of atoms in a chemical formula.
• The mass of one mole of a substance is its relative atomic or formula mass in grams.

Examples

• Calcium (Ca)
  • Relative formula mass: 40
  • Mass of 1 mol: 40 g
• Oxygen (O_2)
  • Relative formula mass: 32
  • Mass of 1 mol: 32 g
• Calcium Carbonate (CaCO_3)
  • Relative formula mass: 100
  • Mass of 1 mol: 100 g

Avogadro Constant

• 6.02 x 10^23 per mole for particles (atoms, molecules, ions).
• Amount in moles applies to various particles.
• Example: 1 mol of sulfur has the same number of particles as 1 mol of sulfur dioxide or sodium ions in sodium chloride.

Calculating Particles

• Number of particles = Avogadro constant x amount of substance in mol.

Example Calculation

• Calculate number of water molecules in 0.5 mol of water:
  • Number of molecules = 6.02 x 10^23 x 0.5
  • Result: 3.01 x 10^23 molecules
• Note: 1 molecule of H2O contains 3 atoms, therefore 3.01 x 10^23 molecules contain 9.03 x 10^23 atoms.

Practice Problem

• Calculate number of oxygen atoms in 0.5 mol of O_2 molecules:
  • Number of atoms = 6.02 x 10^23/mole x 0.5 mole x 2
  • Result: 6.02 x 10^23 atoms

Additional Resources

• Calculations in Chemistry - AQA
• Atom economy, percentage yield and gas calculations - AQA
• Sample exam questions - quantitative chemistry - AQA