Question

The pKa of butyric acid HBut is 4.7. How do you calculate Kb for the butyrate ion But−?

Answer 1

pKb is defined as the negative log of Kb.

We know the sum of pKa of species and pKb of the conjugate base of the same species id equal to pKw at a specific temperature.

i.e.

$pK_w = pK_a(species) + pK_b(conjugate  base)$

At 25° C pKw =14

$14=4.7+pK_b$

$pK_b$=9.3

$pK_b=-logK_b$

$K_b=10^{-9.3}$

$K_b=5.01*10^{-10}$