pH of buffer = pKa + log ([salt]/[acid]) = pKa + log (0.239/0.384) = pKa - 0.206 = 10.32 - 0.206 = 10.114
KOH + KHCO3 K2CO3 + H2O
mole of KHCO3 = 225.0×10-3 × 0.384= 0.0864
Moles of KOH added = 0.0464
Moles of KHCO3 left = 0.0864 - 0.0464 = 0.0400
[KHCO3] = 0.0400/(0.225) = 0.178M
Moles of CO32- = 0.239 × 0.225 + 0.0464 = 0.100
pH of new buffer = pKa + log (0.100/0.0400) = pKa + 0.399
pKa of HCO3- = 10.32 (Ka= 4.8×10-11)
pH = 10.32 + 0.399 = 10.719 = 10.72
For calculating pH of solution we require Ka of acid HCO3- . We can calculate change in pH after adding KOH, because in that case pKa in two equations will cancel out each other.
Change in pH (∆pH) = pka + 0.399 - (pKa - 0.206) = 0.605
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