Question

Selenous acid, H2SeO3, is used in the process of making “blue steel”; at 25°C, H2SeO3has Ka1 = 2.4×10–3 and Ka2 = 4.8×10–8. (Below, H2SeO3 is abbreviated as H2A, and similar abbreviations are made for the conjugate bases) What are the concentrations of H2A, HA–, A2– , H3O+, and OH– in a 1.00 M solution of H2A?

Answer 1

H2A + H2O ----------------> H3O^+ (aq) + HA^- (aq)

I               1                                            0                   0

C             -x                                           +x                 +x

E          1-x                                              +x                +x

        Ka1    = [H^+][HA^-]/[H2A]

     2.4*10^-3   = x*x/1-x

    2.4*10^-3 *(1-x) = x^2

      x   = 0.0478

   [H2A]   = 1-x   = 1-0.0478   = 0.9522M

[H3O^+]     =   x   = 0.0478M

[HA^-]     =   x    = 0.0478M

                    HA^-   + H2O --------------> H3O^+ + A^2-

           Ka2   = [H3O^+][A^2-]/[HA^-]

          4.8*10^-8   = 0.0478*[A^2-]/0.0478

         [A^2-]          = 4.8*10^-8 M >>>>answer