Multiple Choice
A 30.00-mL sample of 0.125 M HCOOH is being titrated with 0.175 M NaOH. What is the pH after 11.2 mL of NaOH has been added? Ka of HCOOH = 1.8 × 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 30.0 mL of NaOH has been added? Ka of HCOOH = 1.8 × 10⁻⁴
A
pH = 3.75
B
pH = 4.25
C
pH = 6.50
D
pH = 5.00
Verified step by step guidance1
Calculate the initial moles of HCOOH using the formula: moles = concentration × volume. Use the given concentration of HCOOH (0.125 M) and its volume (30.00 mL).
Calculate the moles of NaOH added using the formula: moles = concentration × volume. Use the given concentration of NaOH (0.175 M) and its volume (30.0 mL).
Determine the moles of HCOOH that react with NaOH. Since NaOH is a strong base, it will react completely with HCOOH 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, which will be 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: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \). Calculate \( \text{pK}_a \) from \( K_a \) and use the moles of HCOO⁻ and HCOOH to find their concentrations in the total volume (initial volume of HCOOH + volume of NaOH added).
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