Question

a) Methyl Heptenone contains the elements C, H, and O and has a lemon-like odor. The complete combustion of 192 mg of Methyl Heptenone produces 528 mg of CO₂ and 216 mg of H₂O. What are the percent compositions of C, H, and O in this compound?

b) What is the empirical formula for Methyl Heptenone?

Answer 1

Given :

Mass of methyl heptenon = 0.192      g

Mass of CO2= 0.528 g

Molar mass of CO2 = 44.009 g/mol

Mass ratio between CO2 and C is

44.009 : 12.011

We use this mass ratio to calculate mass of carbon present in 0.528 g CO2

Mass of C = 0.528 g x 12.011 g C /44.009 g CO2

                 =0.144 g

Percent composition of C

Percent composition = (Mass of C / total mass of compound) x 100

                                   = (0.144 / 0.192 ) x 100

                                    = 75.1 %

Calculation of moles of C

Mol = mass in g / molar mass

n C = 0.144 g x 1 mol / 12.011 g

       = 1.20E-02 mol

Calculation of moles of H

We use mass of H2O

Mass of H2O formed = 0.216 g                     

Molar mass of H2O is 18.0148 g/mol

Mass ratio between H : H2O is 2.0158 : 18.0148

Mass of H = 0.216 H2O x 2.0158 g H / 18.0148 g H2O

                 = 0.02417 g

Percent composition of H

            = (0.02417 / 0.192 ) x 100

            = 12.6 %

Mol H      = 0.02417 g / 1.0079 g per mol

                 = 0.02398 mol

Calculation of moles of O

We know mass of O = total mass of sample – mass of H – mass of C

= 0.192 g – 0.02417 g – 0.144 g

= 2.37E-02 g

Mass percent of O

= (2.37 E-2/0.192)x 100

=12.4 %

Mol O = 2.37 E-2 g x 1 mol O x 15.998 g

            =1.48E-03       mol

Determination of empirical formula:

We know empirical formula is the smallest whole number ratio of the atoms. To obtain it we have to find smallest whole number of each atom. This is done by dividing all the moles by smallest mol.

Smallest moles among all is of O.

#O

= 1.48E-03 / 1.48E-03

=1

#C

=1.20E-02 /1.48E-03

=8

#H

=0.023980283 / 1.48E-03

=16

Number ratio would be 8C : 16 H : 1O

So the empirical formula is C₈H₁₆O