Final answer:
The rapid change in pH at the start of region V indicates the approach to the equivalence point. During the last step in titrating bicarbonate, the pH of the solution increases and then decreases.
Step-by-step explanation:
During a titration, the rapid change in pH at the start of region V indicates that we are approaching the equivalence point. At this point, the moles of acid and base are equal, resulting in a sudden change in pH. This rapid change occurs because the solution is transitioning from an acidic to a basic pH.
During the last step in titrating bicarbonate, the pH of the solution increases and then decreases. This is because bicarbonate ion (HCO3-) is a weak acid that can act as a buffer. As more base is added, the bicarbonate ion reacts with the base to form the carbonate ion (CO3^2-), causing the pH to increase. However, once all the bicarbonate ion has reacted, the pH begins to decrease because the carbonate ion is a stronger base than the bicarbonate ion.