Question

Terephthalic acid is an important chemical used in the manufacture of polyesters and plasticizers. It contains only C, H, and O. Combustion of 20.40 mg terephthalic acid produces 43.23 mg CO₂ and 6.64 mg H₂O. If 0.271 mole of terephthalic acid has a mass of 45.0 g, determine the molecular formula for terephthalic acid

Answer 1

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

          = 12*43.23*100/(44*20.40)   = 57.8%

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

           = 2*6.64*100/(18*20.40)   = 3.62%

O%    = 100-(C% + H%)

          = 100-(57.8+3.62)   = 38.58%

Element      %           A.Wt                Relative number            simple ratio                    simplest ratio

C              57.8          12                       57.8/12   = 4.82          4.82/2.41   = 2                2*2 = 4

H               3.62           1                        3.62/1    = 3.62          3.62/2.41   = 1.5             1.5*2   = 3

O           38.58           16                      38.58/16   = 2.41           2.41/2.41 = 1                 1*2   = 2

The empirical formula = C4H3O2

empirical formula mass   = 83g/mole

molar mass of terephthalic acid     = mass of terephthalic acid /no of moles of terephthalic acid

                                                     = 45/0.271   = 166g/mole

n   = molar mass/empirical formula mass

      = 166/83 = 2

molecular formula of terephthalic acid    = (empirical formula)n

                                                                 = (C4H3O2)2

                                                                 = C8H6O4

      = 166/