[dolphinsiu]

1990 II 3

The pH value of a 0.02 M ethanedioic acid solution is 1.8. Calculate the concentration of each species present in the solution at 25 degree Celsius.

(Dissociation constants of ethanedioic acid at 25 degree Celsius are :

K1 = 6.5 x 10 ^(-2)

K2 = 6.1 x 10 ^(-5)

In Suggested solution guide, it says that
         H2A    ->      H+       +    HA-
at eqm a          1x 10 ^(-4)        b
         HA- >           H+       +    A2-
at eqm b          1x 10 ^(-4)        c
 
then [H2A]eqm+[HA-]eqm+[A2-]eqm = 0.02 M
        1 x 10 ^(-4)/6.5 x 10 ^(-2)b +b +   6.1 x 10 ^(-5)/1 x 10 ^(-4)b = 0.02M

Why [H+]eqm = 1x 10 ^(-4)?

Why [H2A]eqm+[HA-]eqm+[A2-]eqm = 0.02 M?

Please help me! Please!

 

[Borek]

Why [H+]eqm = 1x 10 ^(-4)?

No idea. It is not true. [H⁺ ] = 10^-1.8 = 0.016

Why [H2A]eqm+[HA-]eqm+[A2-]eqm = 0.02 M?

Mass balance.