Question

What is the concentration of a NH₃ solution having a pH of 11.15?

Answer 1

Answer – Given, pH = 11.15

We know, pOH + pH = 14

So, pOH = 14 –pH

               = 14 – 11.15

               = 2.85

pOH = -log [OH^-]

[OH^-] = 10^-pOH

          = 10^-2.85

         = 0.00141

We know the Kb value for NH₃ =1.8*10^-5

Now we need to put ICE chart

  NH₃ + H₂O <-----> NH₄⁺ + OH^-

I   Y                             0           0

C –x                             +x        +x

E Y-x                        +x       0.00141

So, x = 0.00141 M

We know Kb = [NH₄⁺] [OH^-] / [NH₃]

1.8*10^-5 = 0.00141 *0.00141 / (Y-x)

The value for Kb is small, so we can neglect the x in the Y-x

So, 1.8*10^-5 * Y = 1.99*10^-6

So, Y = 1.99*10^-6 / 1.8*10^-5

          = 0.111 M

The concentration of a NH₃ solution having a pH of 11.15 is 0.111 M