Answer to Question #170703 in Chemistry for Hyeri

Question

What concentration of NH3 is required to make a solution with a pH of 11.22.

Answer 1

Solution:

K_b for NH₃ = 1.8×10^-5

From the pH: pOH = 14 - pH = 14 - 11.22 = 2.78

Hence,

[OH^-] = 10^-pOH = 10^-2.78 = 0.00166

[OH^-] = 0.00166 M

NH₃ + H₂O = NH₄⁺ + OH^-

Hence,

[NH₄⁺] = [OH^-] = 0.00166 M.

[NH₃] = (x - 0.00166) M

K_b = [NH₄⁺][OH^-]/[NH₃]

Hence,

1.8×10^-5 = (0.00166)² / (x - 0.00166)

Assume that x >> 0.00166

1.8×10^-5 = (0.00166)² / (x)

1.8×10^-5x = 2.756×10^-6

x = [NH₃] = 0.153 M

0.153 is >> 0.00166

Answer: [NH₃] = 0.153 M