Question

A buffering solution is made from acetic acid (CH3CO2H) and its conjugate base (CH3CO2–). Which reaction describes the buffering effect of the solution when an acid is added to the solution?
a. CH3CO2H(aq) + OH(aq) → CH3CO2–(aq) + H2O(l)
b. CH3CO2–(aq) + H2O(l) → CH3CO2H(aq) + OH(aq)
c. CH3CO2H(aq) → CH3CO2–(aq) + H+(aq)
d. CH3CO2–(aq) + H+(aq) → CH3CO2H(aq)

Answer 1

Answer:

d. CH3CO2–(aq) + H+(aq) → CH3CO2H(aq)

Explanation:

A buffer is essentially a solution of a weak acid and its conjugate base. The function of a buffer is to maintain the pH of a solution.

The given example represents a buffer system comprising of a weak acid i.e acetic acid CH3COOH and its conjugate base (salt) CH3COO-. The pH is given by the Henderson Hasselbalch equation:

$pH = pKa + log\frac{[CH3COO-]}{[CH3COOH]}-----(1)$

When an acid i.e. H+ ions is added to the buffer then it will combine with the conjugate base to form the weak acid CH3COOH. Therefore, the concentration of the weak acid will increase and correspondingly that of the conjugate base will decrease so as to maintain the pH as depicted in equation (1).

The buffering effect is depicted by the following equation:

CH3CO2–(aq) + H+(aq) → CH3CO2H(aq)