In the titration of a weak acid with a strong base, the pH at the equivalence point is affected mostly by
a.hydrolysis (reaction with water) of the weak acid
b.the dissociation of water molecules
c.the presence of excess base
d.the hydrolysis of the conjugate base of the weak acid
e.the presence of a small amount of weak acid in equilibrium with its salt
Really confused by this question.. please help T^T thanks.
HA + NaOH ==> NaA + H2O is the titration. So at the equivalence point we have NaA which is the conjugate base of HA and you can see that the reverse reaction (which is the hydrolysis) of NaA gives the HA and OH^-. So d.
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To determine the correct answer, let's break down the options and analyze them one by one:
a. Hydrolysis (reaction with water) of the weak acid: Hydrolysis refers to the reaction of a chemical compound with water. In the context of this titration, hydrolysis of the weak acid would not significantly affect the pH at the equivalence point because the weak acid reacts with the strong base, not with water.
b. Dissociation of water molecules: Water spontaneously dissociates into hydronium ions (H3O+) and hydroxide ions (OH-) in a process known as autoionization. However, in this case, the dissociation of water molecules does not have a significant impact on the pH at the equivalence point of the titration.
c. Presence of excess base: Excess base refers to when there is an excess of the strong base added to the solution. While the presence of excess base affects the pH during the titration, it does not specifically impact the pH at the equivalence point. Thus, this option can be eliminated.
d. Hydrolysis of the conjugate base of the weak acid: After the weak acid has been neutralized by the strong base, the remaining solution contains the conjugate base of the weak acid. Hydrolysis of the conjugate base can lead to the formation of hydroxide ions (OH-) and affect the pH. Therefore, this option is a potential contender.
e. Presence of a small amount of weak acid in equilibrium with its salt: This option refers to the concept of a buffer. When a weak acid is titrated with a strong base, as the pH approaches the equivalence point, a buffer system forms consisting of the weak acid and its salt. This buffer system can resist changes in pH. So, this option may also be a strong contender.
In conclusion, the pH at the equivalence point of the titration of a weak acid with a strong base is primarily affected by the hydrolysis of the conjugate base of the weak acid (option d) and the presence of a small amount of weak acid in equilibrium with its salt (option e). Both options contribute to the buffer system formed during the titration, stabilizing the pH.