Multiple Choice
A 25.0 mL sample of 0.20 M HF is titrated with 0.10 M KOH. What is the pH of the solution after the addition of 15.0 mL of KOH? The Ka of HF is 3.5×10^-4.
18. Aqueous Equilibrium
Titrations: Weak Acid-Strong Base
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A 30.00-mL sample of 0.125 M HCOOH is being titrated with 0.175 M NaOH. What is the pH after 21.4 mL of NaOH has been added? Ka of HCOOH = 1.8 × 10⁻⁴
A
3.75
B
4.50
C
5.00
D
4.25
Verified step by step guidance1
Calculate the initial moles of HCOOH using the formula: \( \text{moles} = \text{volume (L)} \times \text{molarity (M)} \). For HCOOH, use 30.00 mL and 0.125 M.
Calculate the moles of NaOH added using the formula: \( \text{moles} = \text{volume (L)} \times \text{molarity (M)} \). For NaOH, use 21.4 mL and 0.175 M.
Determine the moles of HCOOH that react with NaOH. Since NaOH is a strong base and HCOOH is a weak acid, they react in a 1:1 ratio. Subtract the moles of NaOH from the initial moles of HCOOH to find the moles of HCOOH remaining.
Calculate the moles of the conjugate base, HCOO⁻, formed. This is equal to the moles of NaOH added, as each mole of NaOH converts one mole of HCOOH to HCOO⁻.
Use the Henderson-Hasselbalch equation to find the pH: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \( \text{pK}_a = -\log(1.8 \times 10^{-4}) \), \([\text{A}^-]\) is the concentration of HCOO⁻, and \([\text{HA}]\) is the concentration of HCOOH remaining.
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