now let's not get too hasty here. I was performing an estimation, and I believe this would amount to an adequate approximation. I don't believe that we can use the first part of the information given in the problem to calculate the second Ka, not enough information is given and the second aspect of the question certainly does not supplement the first. If it can be done, it'll require some kind of mathematical measure.

Borek, borek, borek...you and HH....you seemed to be obssessed with it w/0 knowing its derivation. It's for buffer solutions, and it is based on the assumption that the conjugate in the form of its salt is added upon the original acid. This is why we can assuming that neither the conjugate or the acid, dissociate...or react with water...it's fundamental general chemistry and you should be familiar with it. The problem isn't even buffer related.

I thought was that the final pH 6.03 would not have been adequately derived from the change in pH due to the first Ka, that is one could have subtracted the former acid concentration given by the first Ka, and the rest would have been due to the influence of the second acid.

Geodome, could you clarify the solution to the problem? I would like to see it.