Which ion is amphiprotic, able to act as both acid and base in solution?

Which ion is amphiprotic, able to act as both acid and base in solution?

• A. OH-
• B. HCO3-
• C. CN-
• D. HSO4-

Answer: (B)

An amphiprotic species can both donate and accept a proton in solution. The bicarbonate ion fits this role because it is the conjugate base of carbonic acid (a weak acid) and the conjugate acid of carbonate (a weak base). This lets it participate in two proton-transfer reactions: it can donate a proton to form carbonate (HCO3- → CO3^2- + H+), acting as an acid; and it can accept a proton to form carbonic acid (HCO3- + H+ → H2CO3), which in water quickly becomes CO2 and H2O, so it also acts as a base. Because of these dual capabilities, bicarbonate is amphiprotic and can buffer pH by shifting in response to the surrounding acidity or basicity. In contrast, OH- is mainly a base in water, while HSO4- is typically viewed as an acid in aqueous solution (though it can technically act as both, its acid behavior is far more dominant here).

An amphiprotic species can both donate and accept a proton in solution. The bicarbonate ion fits this role because it is the conjugate base of carbonic acid (a weak acid) and the conjugate acid of carbonate (a weak base). This lets it participate in two proton-transfer reactions: it can donate a proton to form carbonate (HCO3- → CO3^2- + H+), acting as an acid; and it can accept a proton to form carbonic acid (HCO3- + H+ → H2CO3), which in water quickly becomes CO2 and H2O, so it also acts as a base. Because of these dual capabilities, bicarbonate is amphiprotic and can buffer pH by shifting in response to the surrounding acidity or basicity.

In contrast, OH- is mainly a base in water, while HSO4- is typically viewed as an acid in aqueous solution (though it can technically act as both, its acid behavior is far more dominant here).