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.)

• A. 5.00
• B. 7.00
• C. 8.08
• D. 3.34

Answer: (C)

Verified step by step guidance

1. Calculate the moles of HNO2 in the initial solution using the formula: \( \text{moles} = \text{concentration} \times \text{volume} \). Here, \( \text{volume} \) should be converted to liters.
2. Determine the volume of KOH needed to reach the equivalence point. Since the reaction is a 1:1 stoichiometry, use the moles of HNO2 to find the moles of KOH required, and then calculate the volume using the concentration of KOH.
3. At the equivalence point, all HNO2 has reacted with KOH to form its conjugate base, NO2-. Calculate the concentration of NO2- in the solution by dividing the moles of NO2- by the total volume of the solution at the equivalence point.
4. Use the hydrolysis of the conjugate base NO2- to find the concentration of OH- in the solution. Set up the equilibrium expression for the reaction of NO2- with water, and use the Kb value, which can be found from \( K_w / K_a \), where \( K_w \) is the ion-product constant of water.
5. Calculate the pOH from the concentration of OH- and then convert it to pH using the relationship \( \text{pH} = 14 - \text{pOH} \).