Multiple Choice
The venom of stinging ants contains formic acid, HCOOH, with a dissociation constant (Ka) of 1.8 × 10⁻⁴ at 25 °C. What is the pH of a 0.055 M solution of formic acid?
17. Acid and Base Equilibrium
pH of Weak Acids
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What is the pH change of a 0.220 M solution of benzoic acid (pKa=4.19) if 0.175 M benzoate is added with no change in volume?
A
The pH remains the same
B
The pH decreases to 2.50
C
The pH increases to 4.19
D
The pH decreases to 3.00
Verified step by step guidance1
Identify that the problem involves a buffer solution, which consists of a weak acid (benzoic acid) and its conjugate base (benzoate).
Use the Henderson-Hasselbalch equation to calculate the pH of the buffer solution: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \([\text{A}^-]\) is the concentration of the conjugate base and \([\text{HA}]\) is the concentration of the weak acid.
Substitute the given values into the equation: \( \text{pH} = 4.19 + \log \left( \frac{0.175}{0.220} \right) \).
Calculate the logarithmic term: \( \log \left( \frac{0.175}{0.220} \right) \). This will determine the change in pH from the initial pKa value.
Add the result of the logarithmic term to the pKa value (4.19) to find the new pH of the solution, which indicates the pH change due to the addition of benzoate.
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