Question

The characteristic odor of pineapple is due to ethyl butyrate, a compound containing carbon, hydrogen, and oxygen. Combustion of 3.89 mg of ethyl butyrate produces 8.85 mg of CO2 and 3.61 mg of H2O. What is the empirical formula of the compound?

Answer 1

All the Carbon in Hydrocarbon will completly converted to CO₂

So the number of moles of CO₂ = mass of CO₂ / Molar mass of CO₂

                                               = 8.85x10^-3 g / 44 (g/mol )

                                               = 2.01x10^-4 mol
Each CO₂ has 1 mole of C in it, therefore the number of moles of C = moles CO₂ = 2.01x10^-4 mol

All the Hydrogen present in the given hydrocarbon will ends up into H₂O

So the number of moles of H₂O = mass of H₂O / Molar mass of H₂O

                                              = 3.61x10^-3 g/ 18(g/mol)

                                              = 2.01x10^-4 mol

mass of C = number of moles x molar mass =2.01x10^-4 mol x 12 g/mol = 2.412x10^-3 g = 2.412 mg

Each H₂O has 2 H atoms, so number of moles of H = 2 x number of moles of H₂O

= 2x 2.01x10^-4 mol

= 4.02x10^-4 mol

So mass of H =4.02x10^-4 mol x 1 g/mol = 4.02x10^-4 g = 0.402 mg

So mass of O = mass of sample - ( mass of C + Mass of H)

                      = 3.89 -(2.412+0.402)

= 1.076 mg

Number of moles of O = mass/molar mass = (1.076x10^-3 g) / 16(g/mol) = 0.675x10^-5 mol

The ratio of moles of C , H and O is = 2.01x10^-4 : 4.02x10^-4 : 0.675x10^-4

Divide each number in the ratio by the smallest number we get

                                       = 3 : 6 :1

Therefore the emperical formula is C₃H₆O