Question

Three solutions have the following pH values:

Solution A: pH = 11
Solution B: pH = 10.4
Solution C: pH = 8.2

Which solution has the smallest Kb?

a) A
b) B
c) C
d) Impossible to determine

Answer 1

The trend is that, the larger the value of pKb, the smaller the extent of dissociation. The smaller the extent of dissociation, the less basic (and more acidic) the solution is.

pKb is equal to -log(Kb). Let's compare, just for comparison, Kb = 0.1 and Kb = 0..9

pKb = -log(0.1) = 1
pKb = -log(0.9) = 0.045

This means that the smaller the Kb, the larger the pKb. Consequently, the larger the pKb, the less basic or more acidic the solution is. That would be pH 8.2.

The answer is C.