Of the following, which is the best base to create a buffer with a pH = 8.0?
Aniline, Kb = 4.3 x 10-10
Ethylamine, Kb = 6.4 x 10-4
Hydrazine, Kb = 1.3 x 10-6
Pyridine, Kb = 1.7 x 10-9
The correct answer and explanation is :
To create a buffer solution with a pH of 8.0, we need to select a weak base and its conjugate acid pair that has a pKa close to 8.0. The relationship between pKa and Kb for a base is given by:
[
\text{pKa} = 14 – \text{pKb}
]
where:
[
\text{pKb} = -\log Kb
]
Step 1: Calculate pKa for Each Base
- Aniline:
(\text{pKb} = -\log(4.3 \times 10^{-10}) = 9.37)
(\text{pKa} = 14 – 9.37 = 4.63) - Ethylamine:
(\text{pKb} = -\log(6.4 \times 10^{-4}) = 3.19)
(\text{pKa} = 14 – 3.19 = 10.81) - Hydrazine:
(\text{pKb} = -\log(1.3 \times 10^{-6}) = 5.89)
(\text{pKa} = 14 – 5.89 = 8.11) - Pyridine:
(\text{pKb} = -\log(1.7 \times 10^{-9}) = 8.77)
(\text{pKa} = 14 – 8.77 = 5.23)
Step 2: Choose the Best Base
For an optimal buffer, the pKa of the conjugate acid should be close to the desired pH (8.0). Among the given bases, hydrazine has a pKa of 8.11, which is the closest to 8.0.
Conclusion:
Hydrazine (Kb = 1.3 × 10⁻⁶) is the best choice because its conjugate acid has a pKa of 8.11, which is ideal for maintaining a buffer at pH = 8.0.