Finding concentration from pH and Kb

Question

A certain weak base has a  of ^-7 What concentration of this base will produce a pH of 

Answer 1

Weak base B hydrolyes..

B + H₂O ==> BH⁺ + OH^-

From pH = 10.16, we know pOH = 14-10.16 = 3.84 and OH^- = 1.45x10^-4 M

Kb = 8.40x10^-7 = [BH⁺][OH^-]/[B] = (1.45x10^-4)² / [B]

[B] = 0.0250 M