Question

Which of the following statements about a weak acid strong base titration is/are correct?

• A. The pH after the equivalence point of the weak acid strong base titration is determined by using the $K_b$ expression for the conjugate base.
• B. A buffer solution of weak acid and its conjugate base is formed before the equivalence is reached
• C. The pH at the equivalence point of a weak monoprotic acid strong base titration is equal to the pH at the equivalence point of a strong acid-strong base titration.
• D. The increase in pH in the region near the equivalence The increase in pH in the region near the equivalence point of a weak acid strong base titration is grater than the pH change in the same region of a strong acid strong base titration

Answer 1

Answer: (A), (B)

The correct options are
A The pH after the equivalence point of the weak acid strong base titration is determined by using the $K_b$ expression for the conjugate base.
B A buffer solution of weak acid and its conjugate base is formed before the equivalence is reached

The titration curve is a graph of the volume of titrant, or in our case, the volume of a strong base plotted against the pH. There are several characteristics that are seen in all titration curves of a weak acid with a strong base.

The initial $pH$ (before the addition of any strong base) is higher or less acidic than the titration of a strong acid.

There is a sharp increase in pH at the beginning of the titration. This is because the anion of the weak acid becomes a common ion that reduces the ionization of the acid.

After the sharp increase at the beginning of the titration, the curve only changes gradually. This is because the solution is acting as a buffer. This will continue until the base overcomes the capacity of the buffer.

In the middle of this gradually curve the half-neutralization occurs. At this point, the concentration of weak acid is equal to the concentration of its conjugate base. Therefore the $pH=pKa$. This point is called the half-neutralization because half of the acid has been neutralized.

At the equivalence point, the $pH$ is greater than 7 because all of the acid ($HA$) has been converted to its conjugate base ($A-$) by the addition of $NaOH$ and now the equilibrium moves backwards towards $HA$ and produces hydroxide.

The steep portion of the curve prior to the equivalence point is short. It usually only occurs until a $pH$ of around 10.