Citric Acid (H3C6H5O7) Is A Polyprotic Acid. When Citric Acid Dissolves In Water, Which Is Higher, The Concentration Of H2C6H5O7−Ions Or The Concentration Of HC6H5O72− Ions?
The Correct Answer and Explanation is:
The correct answer is: The concentration of H₂C₆H₅O₇⁻ ions is higher than the concentration of HC₆H₅O₇²⁻ ions.
Explanation:
Citric acid (C₆H₈O₇), with the chemical formula H₃C₆H₅O₇, is a polyprotic acid, meaning it can donate more than one proton (H⁺) per molecule. Specifically, citric acid has three carboxyl groups (-COOH), each capable of losing a proton. The dissociation process occurs in multiple steps:
- First Dissociation (Step 1):
The first proton is donated from the carboxyl group, resulting in the formation of H₂C₆H₅O₇⁻ (the citrate ion):
[
\text{H₃C₆H₅O₇} \rightleftharpoons \text{H₂C₆H₅O₇}⁻ + \text{H}⁺
]
This step has a relatively high dissociation constant (K₁), meaning it is the most likely step to occur when citric acid dissolves in water. As a result, a significant concentration of H₂C₆H₅O₇⁻ ions is present in solution. - Second Dissociation (Step 2):
The second proton is donated from the H₂C₆H₅O₇⁻ ion, forming the HC₆H₅O₇²⁻ ion:
[
\text{H₂C₆H₅O₇}⁻ \rightleftharpoons \text{HC₆H₅O₇}²⁻ + \text{H}⁺
]
This step has a lower dissociation constant (K₂) compared to the first dissociation, indicating that it is less likely to occur. Therefore, fewer HC₆H₅O₇²⁻ ions will be present in solution. - Third Dissociation (Step 3):
The third proton dissociates from HC₆H₅O₇²⁻ to form C₆H₅O₇³⁻, but this step has an even lower dissociation constant (K₃), meaning it occurs to an even lesser extent.
In summary, the dissociation of citric acid predominantly favors the first dissociation step, which produces H₂C₆H₅O₇⁻ ions. Since the second and third dissociations occur to a lesser extent, the concentration of H₂C₆H₅O₇⁻ ions is higher than that of HC₆H₅O₇²⁻ ions in solution.