Question

a certain weak base has a Kb of 7.30 x 10^-7. what concentration of this base will produce a pH of 10.24?

Answer 1

The equilibrium reaction based on a certain weak base B is
B + H2O <=> CA + OH-
EQUILIBRIUM Constant equation:
Kb= [OH-][CA]/[B]
[B]= Base concentration

[CA]= concentration of conjugate acid

Let [OH-]= [CA] = x

14 - pH = pOH

14-10.24 = 3.76 = pOH
-log[OH-] = pOH

-log[OH-]= 3.76
[OH-] = 10^ (-pOH)

[OH-] = 10^-3.76 = 1.737 x 10^-4

[OH -] = [CA] = x

and we now know that x is 1.737 x 10^-4

From the equilibrium constant equation

Kb = x^2/[B]

[B] = x^2/Kb

[B] = (1.737 x 10^-4)^2 / (7.3 × 10-7)

[B]= 0.0413 M