Concept:

(i) A buffer is an aqueous solution designed to maintain a constant pH, even when exposed to small amounts of acids or bases.

(ii)Whether acidic (pH < 7) or basic (pH > 7), a buffer solution consists of a weak acid or base mixed with the salt of its conjugate base or acid, respectively.

(iii)To calculate the specific pH of a given buffer, you need to use the Henderson-Hasselbalch equation for acidic buffers:

\(pH = pka + \log \left( {\frac{{\left[ {A - } \right]}}{{\left[ {HA} \right]}}} \right)\)

Where,Ka is the "dissociation constant" for the weak acid,[A-] is the concentration of conjugate base and [HA] is the concentration of the weak acid.

pH + pOH = 14

Calculation:

According to Henderson equation pH of acidic buffer is:

\(pH = pka + \log \left( {\frac{{\left[ {A - } \right]}}{{\left[ {HA} \right]}}} \right)\)

Here, pka = 4.74, [A-] = 0.02 and [HA] = 0.02

\(pH = 4.74 + \log \left( {\frac{{0.02}}{{0.02}}} \right) = 4.74\)

∵ We know that, pH + pOH = 14

So, pOH = 14 - pH = 14 - 4.74 = 9.26