CHEMISTRY COMMUNITY

For 12.81, the question states to calculate the pH of a diprotic acid solution of H2CO3, ignoring the second deprotonation. In what scenario would we not be allowed to use this approximation and calculate the total [H3O+] for the more accurate pH after both deprotonations?

I think it depends on whether the polyprotic acid is initially a strong acid. Like in the problem in part c, the acid is H2S which is a strong acid, but HS is a weak acid, so in that case you could ignore the second deprotonation because strong acids dissociate almost completely. However, in the case of H2CO3, you wouldn't be able to ignore the second deprotonation because the acid is a weak acid to begin with.