Question

The steroid hormone estriol contains only C, H, and O; combustion analysis of a 3.47 mg sample yields 9.53 mgCO2 and 2.60 mgH2O. On dissolving 7.55 mg of estriol in 0.500 g of camphor, the melting point of camphor is depressed by 1.97 ∘C. [For camphor, Kf = 37.7 (∘C⋅kg)/mol.]

i. What is the molecular weight of estriol?

ii. what is the probable formula?

2) The number of moles of ions in 30.0 mL of 3.00 M Na2SO4 is ________ moles.

Answer 1

i =1 for non electrolyte

Tf   = i*m*Kf

1.97   = 1*m*37.7

m   = 1.97/37.7   = 0.0523 m

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

0.0523      = 7.55*10^-3*1000/G.M.Wt * 0.5

G.M.Wt   = 7.55*10^-3*1000/(0.0523 * 0.5)   = 288.7g/mole

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

           = 12*9.53*100/44*3.47   = 74.9%

H%   = 2*wt of H2O/18* wt of compound

        = 2*2.6*100/18*3.47   = 8.325%

O%   = 100-(C% + H%)

          = 100-(74.9 + 8.325) = 16.775%

Element                       %                   A.Wt              Relative number           simple ratio

C                               74.9                  12                   74.9/12 = 6.24            6.24/1.05    = 6

H                               8.325                 1                    8.325/1   = 8.325        8.325/1.05   = 8

O                               16.775              16                16.775/16    = 1.05       1.05/1.05 = 1

The empirical formula = C6H8O

empirical formula weight (E.F.Wt) = 12*6+ 1*8 + 16*1   = 96

molecular formula =   (empirical formula)n

                      n       = M.Wt/E.F.Wt

                                 = 288.7/96 = 3

molecular formula   = (C6H8O)3 = C18H24O3

2.The number of moles of ions in 30.0 mL of 3.00 M Na2SO4 is ________ moles.

no of moles of Na2So4 = molarity * volume in L

                                         = 3*0.03 = 0.09 moles

Na2So4(aq) --------------------> 2Na^+ (aq) + SO4^2- (aq)

0.09 moles                            2*0.09moles     0.09moles

[Na^+]   = 2*0.09moles    = 0.18 moles

[SO4^2-]    = 0.09moles

                               =