1.23 understand how the formulae of simple compounds can be obtained experimentally, including metal oxides, water and salts containing water of crystallisation

Water of Crystallisation – 

  1. Find the RFM of the compound
  2. Calculate the moles of water lost – mass / RFM 
  3. Calculate the moles of anhydrous salt made – mass/RFM
  4. Work out the ratio of anhydrous salt : water 

Worked example: Hydrated Magnesium sulfate – MgSO4 . 7H20

  1. RFM of MgSO4 . 7H20 = 246
  2. 7 x ((1 x 2) +16 = 18) = 126
  3. (100 x 12 / 246 = 51.2 %

Metal Oxides – 

  1. Weigh compound pre-experiment
  2. Weigh compound after experiment (experiment should have been a reaction of decomposition of metal oxide and salt) 
  3. Compound AB = the original compound, compound B is found by minusing AB-A = B
  4. Now you must weigh compound A and compound B and you can work out the formula – Weight / RFM 

Worked example: Example compound AB

  1. element A = 22g, RFM of 11 – 22/11 = 2
  2. element B = 72g, RFM of 18 – 72 / 18 = 4
  3. A2B4 ( / 2) AB2 
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