Which of the following mixtures gives a buffer solution with pH = 9.5?

Given:

pKb(NH₄OH) = 4.75

Solution:

For a basic buffer:

pOH = pKb + log ([Salt]/[Base])

pH + pOH = 14

pOH = 14 - 9.5 = 4.5

4.5 = 4.75 + log ([Salt]/[Base])

log ([Salt]/[Base]) = -0.25

[Salt]/[Base] = 10^-0.25

[Salt]/[Base] ≈ 0.56

Therefore, the correct mixture is the one containing NH₄OH and NH₄Cl in the ratio 1 : 0.56.

Answer: Mixture of NH₄OH and NH₄Cl forming a basic buffer of pH 9.5.