Multiple Choice
What mass of CH3COONa must be added to create 300.0 mL of a buffer solution with a pH of 5.00 and a CH3COOH concentration of 0.100 M? The pKa of acetic acid is 4.75.
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18. Aqueous Equilibrium
Henderson-Hasselbalch Equation
Multiple Choice
Which of the following correctly represents the Henderson-Hasselbalch equation for a solution of propanoic acid (C2H5COOH) and its conjugate base (C2H5COO^-)?
A
pH = pKa - log([C2H5COOH]/[C2H5COO^-])
B
pH = pKa - log([C2H5COO^-]/[C2H5COOH])
C
pH = pKa + log([C2H5COOH]/[C2H5COO^-])
D
pH = pKa + log([C2H5COO^-]/[C2H5COOH])
Verified step by step guidance1
Recall that the Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the acid and the ratio of the concentrations of its conjugate base and acid.
The general form of the Henderson-Hasselbalch equation is: \(\mathrm{pH} = \mathrm{p}K_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right)\), where \([\text{A}^-]\) is the concentration of the conjugate base and \([\text{HA}]\) is the concentration of the acid.
Identify the acid and conjugate base in this problem: propanoic acid is \(\mathrm{C_2H_5COOH}\) (acid, HA) and its conjugate base is \(\mathrm{C_2H_5COO^-}\) (A\(^-\)).
Substitute the species into the equation: \(\mathrm{pH} = \mathrm{p}K_a + \log \left( \frac{[\mathrm{C_2H_5COO^-}]}{[\mathrm{C_2H_5COOH}]} \right)\).
Compare this form to the options given and confirm that the correct representation has the conjugate base concentration in the numerator and the acid concentration in the denominator, with a positive sign before the logarithm.
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