Final answer:
The pH of a 0.40 M CH₃COOH solution can be calculated using the Ka value and the negative logarithm of the hydronium ion concentration. The addition of sodium acetate leads to an increase in pH due to the common ion effect and buffer formation, as predicted by Le Chatelier's principle.
Step-by-step explanation:
To determine the pH of a 0.40 M CH₃COOH solution, we can use the acid dissociation constant (Ka) given as 1.8 x 10⁻⁵. Setting up the ICE table and solving for the concentration of hydronium ions, we find the pH by taking the negative logarithm of the hydronium ion concentration. The initial pH of the solution can be found using the formula: pH = -log[H₃O+], where [H₃O+] = √(Ka × initial concentration of CH₃COOH).
After the addition of sodium acetate (CH₃COONa), a common ion effect occurs, shifting the equilibrium according to Le Chatelier's principle. This causes the dissociation of CH₃COOH to decrease, reducing the concentration of H₃O+ ions and thus increasing the pH. This can be predicted as the addition of a conjugate base (CH₃COO⁻ from CH₃COONa) will partially neutralize the acid and create a buffer solution.
The pH of a solution containing both 0.40 M CH₃COOH and 0.20 M CH₃COONa would indeed be higher than the pH of a solution with only 0.40 M CH₃COOH. This is due to the buffer's capacity to resist changes in pH upon the addition of small amounts of acid or base.