Question

The foul odor of rancid butter is due largely to butyric acid, a compound containing carbon, hydrogen, and oxygen. combustion analysis of a 4.30-g sample of butyric acid produced 8.59 g co2 and 3.52 g h2o. part a find the empirical formula for butyric acid. express your answer as a chemical formula.

Answer 1

Mass of CO₂ = 8.59 g  

Molar mass of CO₂ = 44 g/mol

Moles of CO₂ = Mass of CO₂ / Molar mass of CO₂ = 8.59 g  / 44 g/mol

                        = 0.1952 moles

Now there is 1 mole of C in 1 mole of CO₂

Therefore, moles of carbon = moles of CO₂ = 0.1952 moles

Mass of carbon = moles of C x  the atomic weight of C

                           = 0.1952 moles x 12 g/mol

                            = 2.3427 g

 Mass of H₂O = 3.52g  

Molar mass of H₂O = 18g/mol

Moles of H₂O =  3.52g /18g/mol = 0.1956 moles

Now there is 2 moles of H in 1 mole of H₂O

Moles of H = 0.1956 moles X 2  = 0.3912 moles

Mass of H = 0.3912 moles X atomic mass of H  

                 = 0.3912 moles X  1 g/mol = 0.3912 g

Mass of butyric acid = 4.30g  

Mass of O = mass of butyric acid - (mass of C + mass of H)

                  = 4.30 g - 2.3427 g - 0.3912 g = 1.5661 g

Moles of O = mass of O / atomic mass of O = 1.5661 g / 16 g/mol

                   = 0.0978 moles  

Divide by smallest mole to get the molar ratio  

C = 0.1952 moles/0.0978 moles  = 2

H =  0.3912 moles/0.0978 moles  = 4

O = 0.0978 moles  / 0.0978 moles  = 1

Empirical formula of butric acid is C₂H₄O  

Molecular formula of butric acid is C₄H₈O₂