Molecular and Empirical Formula of Alkanes

• A molecular formula shows how many atoms of each type are present in a given compound.
• The empirical formula shows the smallest ratio of the different types of atom present.
• The Molecular Formula = M x Empirical Formula (where M= a positive integer).
• The empirical formula of a compound can be worked out from its percentage composition. For a hydrocarbon this can be found out by burning a known mass in oxygen and working out the amounts of carbon dioxide and water formed. This is called combustion analysis.
  Example:

  0.1g of a hydrocarbon x gave 0.309g CO₂ and 0.142g H₂O on combustion. What is the empirical formula of x?

  
  

  Calculate the mass of C and H in 0.1g of the compound (all of the compound goes to H₂O and CO₂, so the mass of carbon and hydrogen in the compound can be worked out from the mass's of carbon dioxide and water produced).
  
  44g CO₂ contains 12g of C (Mr of CO₂ is 44, and Ar of C is 12).
  so 0.309g contains 0.309 x 12/44 = 0.0843g
  
  18g H₂O contains 2g of H
  so 0.142g contains 0.142 x 2/18 = 0.0158g
  
  So the compound contains 0.0843g of Carbon and 0.0158g of Hydrogen.

  
  So the empirical formula of x is C₄H₉.

Useful books for revision:

Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry)
Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)

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