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Cambridge IGCSE Chemistry · 0620
Chapter 3: Stoichiometry — Part 3
Topic 3.3a · The mole and the Avogadro constant
The mole unit
- The mole (mol)
- The SI unit of amount of substance. One mole contains the same number of particles as there are atoms in exactly 12 g of carbon-12.
- Avogadro constant
- The number of particles in one mole of any substance: 6.02 × 1023 (particles may be atoms, molecules, ions, or electrons).
- Molar mass
- The mass of one mole of a substance. Numerically equal to the Mr (or Ar for an element), with units g/mol (or g mol-1). Example: Mr of NaCl = 58.5, so molar mass = 58.5 g/mol.
Basic mole calculations
- Amount from mass
- n = m ÷ Mr (or n = m ÷ molar mass). Rearrangements: m = n × Mr; Mr = m ÷ n.
- Particles from moles
- Number of particles = Avogadro constant × moles = 6.02 × 1023 × n.
- Atoms in molecules
- For molecular substances, first find molecules, then multiply by the number of atoms per molecule. Example: 1 mol of CH4 contains 6.02 × 1023 molecules, each with 5 atoms ? 5.01 × 1024 atoms.
Exam Traps
- Do not confuse moles of molecules with moles of atoms — 1 mol CH4 contains 5 mol of atoms.
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