Question

Which of the following buffer systems would you use if you wanted to prepare a solution having a pH of approximately 9.5?Which of the following buffer systems would you use if you wanted to prepare a solution having a of approximately 9.5?

Answer 1

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!