Moles, relative molecular mass and compounds

1 mole of any compound has a mass equal to the relative molecular mass of that compound expressed in grams. Don't forget that a mole is just another name for an Avogadro number of particles of that substance - 6,02 x 1023.

Example: water H2O

Relative molecular mass = (2 x 1) + 16 = 18
Mass of 1 mole of water = 18g
18g of water contains 6,02 x 1023 molecules of water.

Each molecule of water has two hydrogen atoms and one oxygen atom…….. so 1 mole of water is made up of 2 moles of hydrogen atoms and 1 mole of oxygen atoms*

½ mole of water has a mass of 18 x ½ = 9g **
½ mole of water contains (6,02 x 1023 ) x ½ molecules.

*(of course, the atoms of oxygen and hydrogen are not separate within water but we can still talk about them just the same - it's like saying that twenty people have forty arms and forty legs - the arms and legs are not separate entities but we can still talk about them as if they are)

** ½ can also be written as decimal 0,5. This means that the method for calculation of the mass of a certain number of moles is = the mole fraction x the relative molecular mass.

Homework: Calculate the mass of the following:

  1. 1 mole of calcium carbonate
  2. 0,5 moles of sulphuric acid
  3. 0,2 moles of hydrochloric acid
  4. 5 moles of nitric acid
  5. 0,1 moles of copper sulphate
  6. 0,5 moles of sulphur dioxide
  7. 0,1 moles of magnesium sulphate
  8. 0,01 moles of magnesium chloride
  9. 5 moles of magnesium nitrate
  10. 0,002 moles of sodium chloride

Quote - "If you want to know something, look it up. If you want to understand something, figure it out" - Richard Feynmann