1.6.4 The Mole & the Avogadro Constant

• The Avogadro constant (N_A or L) is the number of particles equivalent to the relative atomic mass or molecular mass of a substance
  • The Avogadro constant applies to atoms, molecules, ions and electrons

• The value of N_A is 6.02 x 10²³ g mol^-1
• The mass of a substance with this number of particles is called a mole (mol)
  • This can be called the molar mass 
  • This is the mass of substance that contains the same number of fundamental units as exactly 12.00g of carbon-12
• The amount / number of moles of a substance, n, the mass of the substance, m, and the molar mass, M, are linked by the equation:

n =

• One mole of any element is equal to the relative atomic mass of that element in grams
  • If you had one mole of carbon in your hand, that is 6.02 x 10²³ atoms of carbon, you would have a mass of 12.00 g
  • One mole of water would have a mass of (2 x 1 + 16) = 18 g

Worked Example

Answers

1. 1. Molar mass of water, H₂O = (2 x 1.0) + 16.0 = 18.0 g mol^-1 
   2. Moles 5.56 moles (to 3 s.f.)
   3. Number of molecules = number of moles x Avogadro's constant = 5.56 x (6.02 x 10²³) = 3.35 x 10²⁴ molecules

Worked Example

Answer 1:

Number of moles 8.31 x10^-16 moles

Mass = moles x molar mass = (8.31 x10^-16) x 195.1 = 1.62 x 10^-13 g (lots of atoms, a tiny mass)

Answer 2:

Molar mass of ethanol, C₂H₅OH = (2 x 12.0) + (5 x 1.0) + 16.0 + 1.0 = 46.0 g mol^-1 

Number of moles 0.0218 moles

Mass = moles x molar mass = 0.0218 x 46.0 = 1.00 g (an exceptionally large number of molecules in just 1 g)

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