Question

You have to prepare a pH 5.00 buffer, and you have the following 0.10 M solutions available: HCOOH (pka=3.74), HCOONa, CH3COOH (pka=4.74), CH3COONa, HCN (pka=9.31), and NaCN. Which solutions would you use?

Answer 1

Answer:

CH3COOH - CH3COONa since its pKa is closest to the required pH.

Explanation:

Hello there!

In this case, in agreement with the theory of buffers as solutions able to withstand severe pH changes due to the addition of acidic or basic substances, it is possible to set up a generation equilibrium expression for the acids herein given:

$Ka=\frac{[A^-][H_3O^+]}{[HA]}$

Which leads to the Henderson-Hasselbach equation:

$pH=pKa+log(\frac{[A^-]}{[HA]} )$

Thus, since all the buffers have [A-]=[HA]=0.10M, the log part becomes 0 and therefore the best buffer will have the closest pKa to the required pH, which is CH3COOH - CH3COONa since its pKa is 4.74.

Best regards!