deprotonation equilibrium for polyprotic acid

[Ruiyu Li 3I]

What does the textbook mean when saying "Estimate the pH of a polyprotic acid for which all deprotonations are weak by using only the first deprotonation equilibrium and assuming that further deprotonation is insignificant"? Does it mean the deprotonation equilibrium for h2co3, for example, is h2co3 + h2o --> hco3- + h3o+ instead of hco3- + 2h2o --> co3 2- + 2h3o+? And what does "An exception is sulfuric acid, the only common polyprotic acid that is a strong acid in its first deprotonation" mean? Does it mean the deprotonation equilibrium for h2so4 is h2so4 + 2h2o --> so4 2- + 2h3o+?

[Jonathan Sautter 1J]

Basically what it is saying is that in weak acids, the deprotonation of the first H+ ion is the only significant contributor to the acids pH because the deprotonation of the second ion is not going to occur very much. Remember that weak acids are incompletely deprotonated and only a very small fraction of the molecules are actually deprotonated. Thus, the fraction of molecules in a weak acid that are going to be deprotonated again for its second H+ ion is going to be so low that it won't play a factor in the weak acids pH. Since sulfuric acid is the only common strong acid that is polyprotic, the first deprotonation is considered completely deprotonated and thus there will be a significant enough amount of deprotonations of the second H+ ion in the weak acid HSO4- for it to affect its pH. The reason that second deprotonation is unlikely in the first place is because the negative charge of the molecule after the first deprotonation attracts the positive charge of the H+ still attached to the molecule, and thus it is extremely hard to separate the positive and negative charges and deprotonate the molecule again.