Question

The sugar fructose contains 40.0% C, 6.7% H, and 53.3% O by mass. A solution of 11.7 g of fructose in 325 g of ethanol has a boiling point of 78.59 °C. The boiling point of ethanol is 78.35 °C, and Kb for ethanol is 1.20 °C kg/mol. What is the molecular formula of fructose?

C (Blank) H (Blank) O (Blank)

Answer 1

DTb = Kb*m

78.59-78.35   = 1.2*m

m                    = 0.24/1.2 = 0.2m

molality    = W*1000/G.M.Wt*weight of solvent

        0.2   = 11.7*1000/G.M.Wt *325

       G.M.Wt   = 11.7*1000/0.2*325   = 180g/mole

    Element            %                     A.Wt            Relative number              simple ratio

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

      H                  6.7                          1                  6.7/1   = 6.7                   6.7/3.33   = 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

                                    = 180/30   = 6

molecular formula   = (CH2O)6 = C6H12O6