CIE A Level Physics (9702) 2019-2021

Revision Notes

16.1.1 The Mole

Amount of Substance

  • 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

The Avogadro Constant

  • In AS Physics, the atomic mass unit (u) was introduced as approximately the mass of a proton or neutron = 1.66 × 10-27 kg
  • This means that an atom or molecule has a mass approximately equal to the number of protons and neutrons it contains
  • A carbon-12 atom has a mass of:

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

  • The exact number for a mole is defined as the number of molecules in exactly 12 g of carbon:

The Avogadro Constant equation 1

  • 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

Mole and the Atomic Mass

  • One mole of any element is equal to the relative atomic mass of that element in grams
    • E.g. Helium has an atomic mass of 4 – this means 1 mole of helium has a mass of 4 g
  • If the substance is a compound, add up the relative atomic masses, for example, water (H2O) is made up of
    • 2 hydrogen atoms (each with atomic mass of 1) and 1 oxygen atom (atomic mass of 16)
    • So, 1 mole of water would have a mass of (2 × 1) + 16 = 18 g

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

Worked Example

How many molecules are there in 6 g of magnesium-24?

Step 1:            Calculate the mass of 1 mole of magnesium

      One mole of any element is equal to the relative atomic mass of that

      element in grams

1 mole = 24 g of magnesium

Step 2:            Calculate the amount of moles in 6 g

The Avogadro Constant equation 3

Step 3:            Convert the moles to number of molecules

1 mole = 6.02 × 1023 molecules

0.25 moles = 0.25 × 6.02 × 1023 = 1.51 × 1023  molecules

Exam Tip

If you want to find out more about the mole, check out the CIE A Level Chemistry revision notes!

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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