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Question

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 carbon dioxide (CO2) and 3.46 mg of water. What is the mass percentage of each element in butyric acid?The molecular mass of butyric acid was determined by experiment to be88. What is the molecular formula?

A
Molecular formula = C4H8O2
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B
Mass percentage of C = 44.35 %, H = 19.06 %, O = 36.59 %
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C
Molecular formula = C3H8O
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D
Mass percentage of C = 54.35 %, H = 9.06 %, O = 36.59 %
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Solution

The correct options are
A Molecular formula = C4H8O2
D Mass percentage of C = 54.35 %, H = 9.06 %, O = 36.59 %
(i) % C=12×8.4544×4.24×100=54.35% %H=2×3.4618×4.24×100=9.06% %O=[10054.359.06]=36.59%

(ii)
% %/at.wt.=Factor Factor/Lowest number
C=54.2 54.212=4.52 4.522.29=2
H=9.2 9.21=9.2 9.22.29=4
O=36.6 36.616=2.29 2.292.29=1
Empirical formula of butyric acid is C2H4O

(iii) Empirical formula mass =44
Molecular mass =88
Molecular massEmpirical mass=8844=2
Moleformula = Empirical formula × n=C4H8O2

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