Multiple Choice
Determine the pH at the equivalence point for the titration of a 40.0 mL sample of 0.100 M HNO2 by 0.200 M KOH. (The pKa of nitrous acid is 3.34.)
18. Aqueous Equilibrium
Titrations: Weak Acid-Strong Base
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What is the approximate pH at the equivalence point of a titration between formic acid (HCOOH) and sodium hydroxide (NaOH), given that 25 mL of formic acid is titrated with 29.80 mL of 0.1567 mol L⁻¹ NaOH? The acid dissociation constant (Ka) for formic acid is 1.8 × 10⁻⁴.
A
pH = 9.5
B
pH = 4.5
C
pH = 7.0
D
pH = 8.2
Verified step by step guidance1
Identify the reaction: Formic acid (HCOOH) reacts with sodium hydroxide (NaOH) to form sodium formate (HCOONa) and water (H₂O). This is a neutralization reaction.
At the equivalence point, all the formic acid has been neutralized by the sodium hydroxide, forming a solution of sodium formate. The pH at this point is determined by the hydrolysis of the formate ion (HCOO⁻).
Calculate the concentration of the formate ion at the equivalence point. Use the initial concentration and volume of NaOH to find the moles of NaOH, which equals the moles of formic acid at the equivalence point.
Use the formula for the hydrolysis of the formate ion: \( \text{HCOO}^- + \text{H}_2\text{O} \rightleftharpoons \text{HCOOH} + \text{OH}^- \). The equilibrium expression is \( K_b = \frac{K_w}{K_a} \), where \( K_w \) is the ion-product constant of water.
Calculate the pH from the concentration of OH⁻ ions produced by the hydrolysis of the formate ion. Use the relation \( pH = 14 - pOH \) to find the pH at the equivalence point.
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