Molecular formula


IB chemistry standard level but also suitable for GCSE and A level chemistry

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Molecular Formula

  • Shows the actual number of atoms of each element in a molecule.
  • Example: a compound contains 40.00% Carbon, 6.73% hydrogen and 53.27% oxygen by mass. 
  • Determine the empirical formula: 
  • C= 40/12.01=3.33  
  • H=6.73/1.01=6.66 
  • O=53.27/16.00= 3.33 
  • Ratio is 3.33:6.66:3.33 = 1:2:1 
  • therefore empirical formula is CH20
  • Molecular formula can be obtained from the empirical formula if the molar mass of the compound is also known
  • CH2O = 12.01+ 1.01+ 1.01+ 16.00 = 30
  • Mr = 60
  • Molecular formula is C2H4O2
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Mel Godfrey

what does mr mean? how do u get mr = 60, after working out that CH2O = 30?

Amritpal Mundra

Mel Godfrey wrote:

mr is molecular mass


where do you get the 12.01 from?!


Mel Godfrey: if you know what the Mr is, in this case it is 60, you can then work out what the molecular formula is. The mr= 60 would have been given in the question

Derryl: 12.01 is the atomic mass of carbon, from the periodic table



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