2.2.1 Amount of Substance

• Amount of substance is the name given when counting the number of particles in a substance
  • Amount of substance is often seen in calculations using the letter / symbol n
  • The units for amount of substance are moles / mol
• Amount of substance links to the Avogadro constant, N_A, which is the number of particles equivalent to the relative atomic, molecular or formula 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 12.00 g 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:

• The molar gas volume is the volume occupied by one mole of any gas, at room temperature and pressure
  • The molar volume is equal to 24 dm³ 
• One mole of any element is equal to the relative atomic mass of the element, in grams
  • For example, if you had one mole of carbon in your hand, you would be holding 6.02 x 10²³ atoms of carbon which have a mass of 12.0 g
  • If you were holding one mole of water, you would be holding 6.02 x 10²³ molecules of water which have a mass of 18.0 g (2 hydrogen + 1 oxygen = (2 x 1.0) + 16.0 = 18.0 g)

Worked Example

Answers

Answer 1:
The molar mass is the mass of one mole of any substance

a. Carbon dioxide, CO₂ = 12.0 + (16.0 x 2) = 44.0 g

b. Magnesium nitrate, Mg(NO₃)₂ = 24.3 + (14.0 x 2) + (16.0 x 3 x 2) = 148.3 g

Answer 2:

One mole of any gas occupies 24.0 dm³ 

a. 36.0 dm³ is 1.5 times the molar gas volume of 24.0 dm³, therefore, there are 1.5 moles of carbon monoxide, CO

b. 9.6 dm³ is 0.4 times the molar gas volume of 24.0 dm³, therefore, there are 0.4 moles of chlorine, Cl₂