Weak Acid and Common Ion Effect

Steps:

• Ethanoic acid (CH3COOH) is a weak acid; in dilute aqueous solution, it partially dissociates, yielding [H+] ≈ 10^{-3} to 10^{-4} M, so pH 3-4.
• Sodium ethanoate (CH3COONa) fully dissociates to provide CH3COO- ions.
• Added CH3COO- is the common ion, shifting the equilibrium CH3COOH ⇌ CH3COO- + H+ leftward (Le Chatelier's principle).
• Reduced dissociation lowers [H+], increasing pH.

Why A is correct:

• For weak acids, initial pH is acidic (3-4); common ion effect suppresses ionization, decreasing [H+] and raising pH per Le Chatelier's principle.

Why the others are wrong:

• B: pH increases, does not decrease, due to suppressed H+ production.
• C: Initial pH is acidic (3-4), not basic (8-9) for a weak acid solution.
• D: Initial pH is not 8-9, and pH increases rather than decreases.

Final answer: A