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 23.0 mL of NaOH has been added? Ka of HCOOH = 1.8 × 10⁻⁴
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18. Aqueous Equilibrium
Titrations: Weak Acid-Strong Base
Multiple Choice
A student titrated a 100 mL sample of 0.100 M acetic acid with 0.050 M NaOH. (For acetic acid, Ka = 1.8 × 10^-5 at room temperature.) Calculate the pH after 50.0 mL of NaOH has been added.
A
4.74
B
8.25
C
7.00
D
3.75
Verified step by step guidance1
Determine the initial moles of acetic acid in the solution using the formula: \( \text{moles} = \text{concentration} \times \text{volume} \). For acetic acid, \( \text{moles} = 0.100 \text{ M} \times 0.100 \text{ L} \).
Calculate the moles of NaOH added using the formula: \( \text{moles} = \text{concentration} \times \text{volume} \). For NaOH, \( \text{moles} = 0.050 \text{ M} \times 0.050 \text{ L} \).
Determine the moles of acetic acid that remain after the reaction with NaOH. Since NaOH reacts with acetic acid in a 1:1 ratio, subtract the moles of NaOH from the initial moles of acetic acid.
Calculate the concentration of acetic acid and acetate ion in the solution after the reaction. The total volume of the solution is now 150 mL (100 mL acetic acid + 50 mL NaOH). Use the formula: \( \text{concentration} = \frac{\text{moles}}{\text{volume}} \).
Use the Henderson-Hasselbalch equation to find the pH: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \( \text{pKa} = -\log(\text{Ka}) \). Calculate \( \text{pKa} \) and substitute the concentrations of acetate ion \([\text{A}^-]\) and acetic acid \([\text{HA}]\) into the equation.
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