On the polyprotic weak acid/strong base titration curve below, label the following points.
The point where the pH corresponds to a solution of H₂A in water.
b) The point where the pH corresponds to a solution of HA” in water.
) The point where the pH corresponds to a solution of A³ in water. c
d) The point where pH = pKa
e) The point where pH = pK_a2.
The Correct Answer and Explanation is :
- The point where the pH corresponds to a solution of H₂A in water: This corresponds to the initial point in the titration before any base is added. At this point, the solution contains only the weak acid (H₂A), which can donate protons (H⁺) to the solution. You will typically observe a relatively low pH because H₂A partially dissociates in water.
- The point where the pH corresponds to a solution of HA⁻ in water: This occurs after the first dissociation of H₂A has happened but before the second dissociation. The solution now contains the conjugate base of the weak acid, HA⁻. HA⁻ is a weaker base compared to A²⁻, but it can still accept protons. The pH at this point will be higher than the initial pH of the H₂A solution because HA⁻ raises the pH by absorbing H⁺ ions.
- The point where the pH corresponds to a solution of A²⁻ in water: This point comes after the second dissociation of the acid. The solution now contains A²⁻, the fully deprotonated form of the acid. A²⁻ is a stronger base than HA⁻ and will absorb protons from the water, significantly raising the pH.
- The point where pH = pKa₁: This occurs at the first equivalence point, where the concentration of H₂A is equal to HA⁻. At this point, the pH of the solution is roughly equal to the pKa₁ (the first dissociation constant) because the weak acid has been partially converted to its conjugate base, and the system is at a buffering point.
- The point where pH = pKa₂: This corresponds to the second equivalence point, where the concentration of HA⁻ is equal to A²⁻. At this point, the pH is approximately equal to the pKa₂, the second dissociation constant, as the weak acid has been fully converted to its conjugate base, and the system is buffering again, but this time at a higher pH.
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