CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Benzoic acid, C6H5COOH, has an acid-dissociation constant of Ka=6.3×10−5. A very dilute benzoic-acid solution is prepared with an initial concentration of 8.0×10−4M at 25 °C.
(a) Using the approximation that the equilibrium concentration of benzoic acid is effectively equal to its initial concentration, calculate the equilibrium [H3O+] and the pH of the solution.
(b) Without making the approximation used in part (a), determine the exact equilibrium [H3O+] and pH by solving the appropriate equilibrium expression algebraically.
(c) Calculate the percent ionization of benzoic acid using the result from part (b).
(d) Compare the pH values obtained in parts (a) and (b). Explain which value is more reliable for this solution and why the other is less accurate.