Question

Lactic acid, the substance that builds up in muscles and causes them to “burn” when they are worked hard, has a molar mass of 90amu. If it is 40.0% C, 6.71% H and 53.3% O, what is the molecular formula of lactic acid?

            a) CH₂O              b) C₂H₄O₂                 c) C₃H₃O₃               d) C₃H₆O₃           e) C₆HO₈

Answer 1

          Element           %            A.Wt            Relative number          simple ratio

          C                  40               12                 40/12 = 3.33            3.33/3.33 = 1

          H                  6.71               1                6.71/1 = 6.71          6.71/3.33   = 2

         O                  53.3             16               53.3/16 = 3.33           3.33/3.33   = 1

                   Empirical formula = CH2O

               molecular formula = (empirical formula)n

                                 n           = M.Wt/E.F.Wt

                                             = 90/30 = 2

                   molecular formula = (CH2O)2 = C2H4O2

b) C₂H₄O2