The following pictures represent solutions that contain one or more of the compounds H2A, NaHA, and Na2A, where H2A is a weak diprotic acid. (Na+ ions and solvent water molecules have been omitted for clarity.)

(a) Which of the solutions are buffer solutions?

Verified step by step guidance

1. Identify the components of a buffer solution, which typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid.
2. Examine solution A: It contains both H2A (weak acid) and HA^- (conjugate base), which is a characteristic of a buffer solution.
3. Examine solution B: It contains H2A, HA^-, and A^2-. The presence of both H2A and HA^- suggests it can act as a buffer solution.
4. Examine solution C: It contains only HA^- and A^2-. This does not fit the typical buffer composition of a weak acid and its conjugate base.
5. Examine solution D: It contains only A^2-, which is not a buffer solution as it lacks the weak acid component.

Key Concepts

Buffer Solutions

Buffer solutions are mixtures that can resist changes in pH upon the addition of small amounts of acid or base. They typically consist of a weak acid and its conjugate base, or a weak base and its conjugate acid. In this case, H2A (the weak diprotic acid) and its deprotonated forms (HA- and A2-) can form a buffer system, allowing the solution to maintain a relatively stable pH.

Weak Diprotic Acids

A weak diprotic acid, like H2A, can donate two protons (H+) in a stepwise manner. The first dissociation produces HA-, and the second dissociation produces A2-. The presence of both the acid and its conjugate base in solution is essential for buffer action, as they can neutralize added acids or bases, thus stabilizing the pH.

Equilibrium in Acid-Base Reactions

In acid-base chemistry, equilibrium refers to the state where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. For buffer solutions, the equilibrium between the weak acid and its conjugate base is crucial, as it determines the solution's ability to resist pH changes. Understanding this equilibrium helps identify which solutions can act as buffers.