Question

QUESTION 22

± pH and Percent Ionization of a Weak Base

The degree to which a weak base dissociates is given by the base-ionization constant, Kb. For the generic weak base, B

B(aq)+H2O(l)⇌BH+(aq)+OH−(aq)

this constant is given by

Kb=[BH+][OH−][B]

Strong bases will have a higher Kb value. Similarly, strong bases will have a higher percent ionization value.

Percent ionization=[OH−] equilibrium[B] initial×100%

Strong bases, for which Kb is very large, ionize completely (100%). For weak bases, the percent ionization changes with concentration. The more dilute the solution, the greater the percent ionization.

Ammonia, NH3, is a weak base with a Kb value of 1.8×10−5.

PART A

Part A

What is the pH of a 0.280 M ammonia solution?

Express your answer numerically to two decimal places.

PART B

What is the percent ionization of ammonia at this concentration?

Express your answer with the appropriate units.

Answer 1

part A)

concnetration of NH3 = 0.280 M

Kb = 1.8 x 10^-5

[OH-] = sqrt (Kb x C)

         = sqrt (1.8 x 10^-5 x 0.280)

[OH-] = 2.245 x 10^-3 M

pOH = -log [OH-] = -log (2.24 x 10^-3)

pOH = 2.65

pH = 11.35

part B)

% ionization = ([OH-] / C ) x 100

                     = (2.245 x 10^-3 / 0.280 )x 100

                     = 0.802 %