Answer:
b. 0,08M NH₄⁺ / 0,12M NH₃
Explanation:
The buffers are:
a. 0,08M H₂PO₄⁻ / 0,12M HPO₄²⁻
b. 0,08M NH₄⁺ / 0,12M NH₃
It is possible to find out the pH of a buffer using Henderson-Hasselbalch formula:
pH = pka + log₁₀ [A⁻] / [HA] (1)
Where A⁻ is the conjugate base of the weak acid, HA.
a. For the system H₂PO₄⁻ / HPO₄²⁻ pka is 7,198. Replacing in (1)
pH = 7,198 + log₁₀ [0,12] / [0,08]
pH = 7,37
That means this buffer system don't have a pH of approximately 9,5
b. For the system NH₄⁺ / NH₃ pka is 9,3. Replacing in (1)
pH = 9,3 + log₁₀ [0,12] / [0,08]
pH = 9,50
That means this buffer system is the buffer you need to use.
I hope it helps!
