Question

Calculate the pH at 25

°

C of a 0.13 M solution of a weak base with a K_b of 3.3

×

10⁻¹¹.

Answer 1

Answer

pH = 8.32

Explanation

If the weak base sympolisd as B, the dissociation equation is

B(aq) + H2O(l)  <------> BH⁺(aq) + OH^-(aq)

K_b = [BH⁺][OH^-]/[B] = 3.3×10^-11

Initial concentration

[B] = 0.13

[BH⁺] = 0

[OH^-] = 0

Change in concentration

[B] = -x

[BH⁺] = +x

[OH^-] = +x

Equillibrium concentration

[B] = 0.13 - x

[BH⁺] = x

[OH^-] = x

x²/(0.13 -x) = 3.3×10^-11

solving for x

x = 2.0712×10^-6

[OH^-] = 2.0712×10^-6M

pOH = 5.68

pH = 14 - pOH

pH = 14 - 5.68 = 8.32