Question
Mon May 07, 2012 By:
 

Butyric acid contains only C,H and O. A 4.24 mg sample of butyric acid is completely burned. It gives 8.45 mg of CO2 & 3.46mg of H2O. The molecular mass of butyric acid was determined by experiment to be 88 amu. What is molecular formula? It's my hereby request not to just give the answer, as it is already given in book. But since I do not know how to solve it. so please give the complete solution. Thanx.

Expert Reply
Thu May 10, 2012
In CO2 the no of C atom is 1.
it means 44 mg of CO2  = 12 mg C
and 8.45 mg  = 12 X 8.45 / 44
                    =2.30mg
% of C = 2.30 X 100  / 4.24
           = 54.3 %
Similarly in H2O = 2H
               in 3.46mg of H2O H = 2 X 3.46 /18
                = 0.384mg
% of H = 0.348 x100 / 4.24  =9 %
 % of O = 100 - 63.3
 = 36.7%
By calculation we get the simple whole no ratio = 2:4:1
so empirical formula is C2H4O
and the value of n= 88 /44 =2
so molecular formula = c4 H8 O2
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