Work out base dissociation constant instantly with clear inputs, formula shown and shareable results.
A weak base is handled through pOH rather than pH: pOH = pKw - pH gives the hydroxide concentration, which equals the protonated base formed. Kb is then [OH-]^2 over the base remaining. A 0.1 M base reading pH 11.13 has [OH-] = 1.35e-3 and Kb near 1.8e-5, matching ammonia.
Equilibrium expression
Kb = [OH-][BH+] / [B] = [OH-]^2 / (C - [OH-])
Hydroxide from pH
[OH-] = 10^-(pKw - pH)
Because a base's equilibrium is written in terms of hydroxide production. Converting through pKw keeps the expression honest.
The method breaks down for strong bases: they ionise fully, so C - [OH-] approaches zero and Kb is effectively infinite.