Question

A certain weak base has a Kb of 8.70 × 10^-7. What concentration of this base will produce a pH of 10.31?

Answer 1

pH = 10.31

pOH = 14 - pH = 14.10.31 = 3.69

pOH = 3.69

[OH^-] = 10^-pOH  = 10^-3.69 = 2.04 10^-4

weak base dissociated as

B + H₂O ⇌ BH⁺ + OH^-

K_b for weak base

Kb = [BH⁺][OH^-]/[B]

[BH⁺] = [OH^-] = 2.04 10^-4

Kb = ( 2.04 10^-4 ) ( 2.04 10^-4 ) /[B]

8.70 10^-7 =  ( 2.04 10^-4 ) ( 2.04 10^-4 ) /[B]

[B] = ( 2.04 10^-4 ) ( 2.04 10^-4 ) / (8.70 10^-7 )

[B] = 0.04783 M

concentration of weak base at pH 10.31 is = 0.04783 M