Question

A compound containing only C, H, and O, was extracted from the bark of the sassafras tree. The combustion of 34.7 mg produced 94.2 mg of CO2 and 19.3 mg of H2O. The molar mass of the compound was 162 g/mol. Determine its empirical and molecular formulas.

Answer 1

C%    = 12*wt of CO2*100/44*wt of compound

           = 12*94.2*100/44*34.7    = 74.04%

H%     = 2*wt of H2O*100/18*wt of compound

           = 2*19.3*100/18*34.7    = 6.17%

O      = 100-(C% + H%)

         = 100-(74.04+6.18)   = 19.78%

element              %                  A. Wt            relative number                   simple ratio

C                      74.04                           12                                 74.04/12   = 6.17                 6.17/1.236 = 5

H                       6.17                            1                 6.17/1 = 6.17                   6.17/1.236 = 5

O    19.78 16 19.78/16 = 1.236          1.236/1.236   = 1

         The empirical formula = C5H5O

The empirical formula mass = 81g/mole

The molar mass of the compound = 162 g/mol

molecular formula   = (empirical formula)n

                    n           = molar mass of the compound/ empirical formula mass

                                   = 162/81 = 2

molecular formula   = (C5H5O)2 = C10H10O2