Calculate pKb and Kb for sodium benzoate given that benzoic acid has pKa value equal to 4.20 at 25 °C.

Sodium benzoate ( molecular weight = 144.11 g/ mole ) completely dissociates in water to produce sodium and benzoate ions. Benzoate ions accept protons from water to produce benzoic acid and hydroxyl ions. Therefore, it acts as a weak base in solution. Its pKb and Kb values can be calculated from pKa of benzoic acid ( 4.20 at 25 °C) as the following:

pKw = pKa + pKb

14.00 = 4.20 + pKb

pKb = 14.00 - 4.20

pKb = 9.80

pKb = -log Kb

Kb = 10^-pKb

 Kb = 10^-9.80

Kb = 1.59×10^-10

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