At very high pH, which species dominates for a polyprotic acid?

At very high pH, which species dominates for a polyprotic acid?

• A. H3A
• B. HA-
• C. A2-
• D. H2A

Answer: (C)

For a polyprotic acid, increasing pH drives deprotonation step by step. The species present shift toward the form with more protons removed as the environment becomes more basic: H3A → H2A− → HA2− → A3−. At very high pH, the most deprotonated form available will dominate. Among the given options, the species with two protons removed (the most deprotonated form shown) is the one that dominates. In a full picture, the fully deprotonated form A3− would win at extremely high pH, but since that form isn’t listed, the next most deprotonated option, A2−, is the best answer.

For a polyprotic acid, increasing pH drives deprotonation step by step. The species present shift toward the form with more protons removed as the environment becomes more basic: H3A → H2A− → HA2− → A3−. At very high pH, the most deprotonated form available will dominate. Among the given options, the species with two protons removed (the most deprotonated form shown) is the one that dominates. In a full picture, the fully deprotonated form A3− would win at extremely high pH, but since that form isn’t listed, the next most deprotonated option, A2−, is the best answer.