Multiple Choice
Consider the titration of 25.00 mL of 0.150 M hydrazoic acid (Ka = 1.90 × 10^-5) with 0.200 M NaOH. What is the pH of the solution after the addition of 15.00 mL of NaOH solution?
18. Aqueous Equilibrium
Titrations: Weak Acid-Strong Base
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In a titration of a weak acid with a strong base, what determines the pH at the equivalence point?
A
The volume of the weak acid added
B
The concentration of the strong base
C
The initial concentration of the weak acid
D
The hydrolysis of the conjugate base of the weak acid
Verified step by step guidance1
Understand that in a titration of a weak acid with a strong base, the equivalence point is reached when the moles of the base added equal the moles of the acid initially present.
At the equivalence point, the weak acid has been completely converted into its conjugate base. This means the solution contains the conjugate base and water.
The pH at the equivalence point is determined by the hydrolysis of the conjugate base. This is because the conjugate base can react with water to form hydroxide ions, which affects the pH.
Write the hydrolysis reaction of the conjugate base: \( \text{A}^- + \text{H}_2\text{O} \rightleftharpoons \text{HA} + \text{OH}^- \). This reaction shows how the conjugate base can increase the pH by producing \( \text{OH}^- \) ions.
Calculate the pH by determining the concentration of \( \text{OH}^- \) produced from the hydrolysis and then using the relation \( \text{pH} = 14 - \text{pOH} \), where \( \text{pOH} = -\log[\text{OH}^-] \).
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