Correct Answer - Option 3 : 9.26
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
