OCR A Level Physics

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5.2.1 The Avogadro Constant

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Avogadro's Constant

  • The atomic mass unit (u) is approximately the mass of a proton or neutron = 1.66 × 10-27 kg
  • This means that the mass of an atom or molecule can be calculated using the number of protons and neutrons it contains
    • For example, a carbon−12 atom has a mass of:

12 u = 12 × 1.66 × 10-27 = 1.99 × 10-26 kg

The Mole

  • In thermodynamics, the amount of substance is measured in the SI unit ‘mole’
    • This has the symbol mol
    • The mole is a unit of substance, not a unit of mass

  • The mole is defined as:

The SI base unit of an ‘amount of substance’. It is the amount containing as many particles (e.g., atoms or molecules) as there are atoms in 12 g of carbon-12

  • The mole is an important unit in thermodynamics
  • If we consider the number of moles of two different gases under the same conditions, their physical properties are the same

  • One mole of a substance is defined as the number of molecules in exactly 12 g of carbon:

Mole Equation_2

Avogadro's Constant

Avogadro’s constant (NA) is defined as:

 The number of atoms of carbon-12 in 12 g of carbon-12; equal to 6.02 × 1023 mol−1

  • For example, 1 mole of sodium (Na) contains 6.02 × 1023 atoms of sodium
  • The number of atoms can be determined if the number of moles is known by multiplying by NA.
    • For example: 2.0 mol of nitrogen contains:  2.0 × NA = 2.0 × 6.02 × 1023 = 1.20 × 1024 atoms

Molar Mass

  • The molar mass of a substance is the mass, in grams, in one mole
    • Its unit is g mol−1

  • The number of moles from this can be calculated using the equation:

The Avogadro Constant equation 2

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