[erhernandez 02]

I learned that polyprotic acids can have multiple Ka' s and that when this type of acid is dissolved in water, the dissociation of the first hydrogen contributes more hydrogen (or hydronium ions) than the second. Hence, when calculating the concentration of H3O+ we only need the first Ka because the contribution from the others is insignificant.

However, sulfuric acid seems to be an exception. Being a strong acid, it fully dissociates. When doing calculations, you have to consider both the first and second ka of sulfuric acid. My question is, why? I figured that since it is a strong acid and because it is a polyprotic acid I would only need the first ka (which is just the initial concentration of sulfuric acid). The second should be insignificant. Why is it not?

[Borek]

Hence, when calculating the concentration of H3O+ we only need the first Ka because the contribution from the others is insignificant.

That's not true - so the rest is a moot.

If the difference between both dissociation constants is high enough, and the second dissociation constant is not too low, then yes, it can be ignored. But it is just a rule of thumb, and rules of thumb - if treated too literally - are dangerous.