1.19 carry out mole calculations using relative atomic mass (Ar) and relative formula mass (Mr)

The mole enables us to count atoms and molecules by weighing. The relative atomic mass scale is deliberately set so that if we weigh out the relative atomic mass of an atom in grams, we have a mole of those atoms.

e.g.      carbon has a relative atomic mass of 12

if we weigh out 12 g of carbon, we have one mole of carbon atoms

It stands to reason that if we weigh out 24 g of carbon, we would have two moles of carbon atoms and so on. The important relationship between moles and mass is given by:

MOLES = MASS ÷ MOLAR MASS        (molar mass is the mass of a mole e.g. Mr or Ar)

MASS = MOLES x MOLAR MASS

e.g. 1    what is the mass of 0.25 mol of H2O?

H2O has a molar mass (relative formula mass) of:             (2 x 1) + 16 = 18

Mass    = moles × molar mass

= 0.25 x 18 = 4.5 g

e.g. 2    how many moles are in 1.7 g of NH3?

            NH3 has a molar mass (relative formula mass) of:  14 + (3 x 1) = 17

            moles   = mass ÷ molar mass

= 1.7 ÷ 17 = 0.10 mol