Multiple Choice
What is the pH of a 0.750 M solution of KH2PO4, considering it is a weak acid?
17. Acid and Base Equilibrium
pH of Weak Acids
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What is the pH of the solution at the halfway point of the titration of acetic acid with sodium hydroxide?
A
pH = 7.00
B
pH = 4.76
C
pH = 2.38
D
pH = 9.24
Verified step by step guidance1
Identify that the problem involves the titration of a weak acid (acetic acid) with a strong base (sodium hydroxide). At the halfway point of the titration, half of the acetic acid has been converted to its conjugate base, acetate.
Recall that at the halfway point of a titration of a weak acid with a strong base, the concentration of the weak acid equals the concentration of its conjugate base. This is a key point where the pH equals the pKa of the weak acid.
Use the Henderson-Hasselbalch equation to find the pH at the halfway point: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \). At the halfway point, \([\text{A}^-] = [\text{HA}]\), so the equation simplifies to \( \text{pH} = \text{pKa} \).
Determine the pKa of acetic acid. The pKa is a known constant for acetic acid, which is approximately 4.76.
Conclude that at the halfway point of the titration, the pH of the solution is equal to the pKa of acetic acid, which is 4.76.
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