Multiple Choice
A 1.00 L buffer solution is 0.250 M in HF and 0.250 M in NaF. Calculate the pH of the solution after the addition of 0.150 moles of solid NaOH. Assume no volume change upon the addition of base. The Ka for HF is 3.5 × 10^-4.
18. Aqueous Equilibrium
Intro to Buffers
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A buffer contains the weak acid HA and its conjugate base A⁻. The weak acid has a pKa of 5.25 and the buffer has a pH of 4.82. Which statement is true of the relative concentrations of the weak acid and the conjugate base in the buffer?
A
The concentrations of HA and A⁻ are unrelated to the pH and pKa.
B
The concentration of HA is less than the concentration of A⁻.
C
The concentration of HA is equal to the concentration of A⁻.
D
The concentration of HA is greater than the concentration of A⁻.
Verified step by step guidance1
To determine the relative concentrations of the weak acid (HA) and its conjugate base (A⁻) in the buffer, we can use the Henderson-Hasselbalch equation, which is:
Given that the pH of the buffer is 4.82 and the pKa of the weak acid is 5.25, we can substitute these values into the Henderson-Hasselbalch equation:
Rearrange the equation to solve for the ratio of the concentrations of A⁻ to HA:
Calculate the difference between the pH and pKa: . This negative value indicates that the concentration of HA is greater than the concentration of A⁻.
Since the logarithm of the ratio is negative, it implies that the ratio is less than 1, meaning the concentration of HA is greater than the concentration of A⁻.
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