Question

A solution is prepared by adding 100 mL of 1.0 M HC₂H₃O₂(aq) to 100 mL of 1.0 M NaC₂H₃O₂(aq). The solution is stirred and its pH is measured to be 4.73. After 3 drops of 1.0 M HCl are added to the solution, the pH of the solution is measured and is still 4.73. Which of the following equations represents the chemical reaction that accounts for the fact that acid was added but there was no detectable change in pH?
(A) $H_3O^+(aq) + OH^-(aq) \rightarrow 2 H_2O(l)$
(B) $H_3O^+(aq) + Cl^-(aq) \rightarrow HCl(g) + H_2O(l)$
(C) $H_3O^+(aq) + C_2H_3O_2^-(aq) \rightarrow HC_2H_3O_2(aq) + H_2O(l)$
(D) $H_3O^+(aq) + HC_2H_3O_2(aq) \rightarrow H_2C_2H_3O^+(aq) + H_2O(l)$

Answer 1

Answer: Option (c) is the correct answer.

Explanation:

When an acid or base is added to a solution then any resistance by the solution in changing the pH of the solution is known as a buffer.

This is because a buffer has the ability to not get affected by the addition of small amounts of an acid or a base. As a result, it helps in maintaining the pH of the solution.

In the give case, when we add the HCl then more number of protons will dissociate. This causes the acetate to react with the protons and leads to the formation of acetic acid.

We know that acetic acid is a weak acid and it dissociates partially or feebly. Therefore, no change in pH will take place.

Thus, we can conclude that equation $H_{3}O^{+}(aq) + C_{2}H_{3}O^{-}_{2}(aq) \rightarrow HC_{2}H_{3}O_{2}(aq) + H_{2}O(l)$ represents the chemical reaction that accounts for the fact that acid was added but there was no detectable change in pH.