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Calculating Relative Formula Mass

Calculating Relative Formula Mass

 

H2 H x 2 2 x 1 = 2
Ca(OH)2 (1 x Ca) + (2 x O) + (2 x H) 40 + (2 x 16) + (2 x 1) = 74

Straightforward stuff.

 

Amount in moles = Mass of Substance (g) /RFM of Element or Compound (g)

Calculations using moles:

The equation for sodium chloride is:               2Na + Cl2 -> 2NaCl

 

If 2.3g of Na was used:

  1. Find out how many moles of Chlorine was used
  2. Find out the volume of Chlorine used in the reaction
  3. Find out the mass of sodium chloride produced

 

  1. Firstly, convert the grams of Na into moles:

2.3 / 23g = 0.1 mol

 

The equation says that 2 moles of Na and 1 mole of Cl (1 mole of a diatomic molecule is always X2) is needed to produce 2 moles of NaCl, so if 0.1 mol of Na is used, then half of that is the amount of chlorine used in the reaction in moles.

 

So moles of Cl used = 0.1 / 2 = 0.05 mol

 

  1. One mole of any gas has a volume of 24 dm3 (24000cm3) at room temperature and pressure. This is also called the molar volume.

 

Cl2 is a gas and the moles used in the reaction = 0.05 mol

So the volume of Cl2 gas used = 0.05 x 24000 = 1200cm3

 

  1. The moles of NaCl produced is 0.1 mol (if 2 moles of Na gives 2 moles of NaCl, then 0.1 mole of Na will give 0.1 mole of NaCl). So all you do is:
  2. Find the RFM of NaCl (58.5)
  3. Multiply that by 0.1 (5.85g)