Answer:
<em>a. </em>The buffering range is between 2.74 and 4.74.
<em>b.</em> The ratio of the formate to the formic acid is 10.23.
Explanation:
<em>a.</em> For every buffer solution, the optimal effective range is pH = pKa ± 1. Outside this range, the buffer does not work properly.
For the formic acid, the pKa is 3.74, thus the optimal range is between 2.74 and 4.74.
<em>b. </em>The Henderson-Hasselbalch equation is a chemical expression used to calculate the pH of a buffer knowing the ratio of the acid to base, or to calculate the ratio knowing the pH. The expression is:
![pH = pKa + Log \frac{[A^{-}]}{[HA]}](https://tex.z-dn.net/?f=pH%20%3D%20pKa%20%2B%20Log%20%5Cfrac%7B%5BA%5E%7B-%7D%5D%7D%7B%5BHA%5D%7D)
where [A^{-}] is the concentration of the conjugate base and [HA] is the concentration of the acid.
For a formic acid/potassium formate solution that has a pH of 4.75 and pka of 3.74:
![pH - pKa =4.75 - 3.74 = 1.01 = Log \frac{[A^{-}]}{[HA]}](https://tex.z-dn.net/?f=pH%20-%20pKa%20%3D4.75%20-%203.74%20%3D%201.01%20%3D%20Log%20%5Cfrac%7B%5BA%5E%7B-%7D%5D%7D%7B%5BHA%5D%7D)
![\frac{[A^{-}]}{[HA]} = 10^{1.01} = 10.23](https://tex.z-dn.net/?f=%5Cfrac%7B%5BA%5E%7B-%7D%5D%7D%7B%5BHA%5D%7D%20%3D%2010%5E%7B1.01%7D%20%20%3D%2010.23)