Question

Calculate the pH and moralities of all the species in a 0.150 M ammonia solution. Ka = 5.55 x 10-10

Answer 1

Given :

[NH3] = 0.150 M

Ka = 5.55 E-10

Solution:

Kb is calculated as

Kb = 1.0 E-14 /ka = 1.0 E-14/5.55 E-10 = 1.8 E-5

ICE

                NH3      +            H2O   ----- > NH4+   + OH-

I               0.150-x                                                 0              0

C             -x                                                            +x           +x

E              (0.150-x)                                              x              x

Ka = 1.8E-5 = x^2 / 0.150           since ka is very small we can neglect x in the denominator.

x = 1.644 E-3

[OH-] = 1.644E-3 M

pOH = - log (1.644 E-3 ) = 2.87

pH=14-2.87 = 11.2

pH = 11.2