The Avogadro Constant (OCR A Level Physics): Revision Note

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:

Avogadro's Constant

Avogadro’s constant (N_A) 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 N_A.

  • For example: 2.0 mol of nitrogen contains:  2.0 × N_A = 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: