chemistry help q

Question

A certain weak base has a K b of 7.90x10^-7. What concentration of this base will produce a pH of 10.30?

Answer 1

Let the weak base be B. In water, we have...

B + H₂O ==> BH⁺ + OH^-

Kb = [BH⁺][OH^-]/[B] = 7.90x10^-7

From the pH = 10.30, we can find the pOH and hence the [OH-].

.....pOH = 14 - pH = 14 - 10.30 = 3.70

.....[OH-] = 1x10^-3.70 = 2x10^-4 M

Kb = 7.9x10^-7 = (2x10^-4)(2x10^-4) / [B]

[B] = 4x10^-8 / 7.9x10^-7

[B] = 0.051 M