± pH and Percent Ionization of a Weak Base The degree to which a weak base disso

Question

± 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 1.8×105.
Part A
What is the pH of a 0.175 M ammonia solution?
Express your answer numerically to two decimal places.

pH =

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Part B
What is the percent ionization of ammonia at this concentration?
Express your answer with the appropriate units.

% ionization =

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Explanation / Answer

part A :

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

0.175 0 0

0.175 - x x x

Kb = x^2 / 0.175 - x

1.8 x 10^-5 = x^2 / 0.175 - x

x = 1.77 x 10^-3

[OH-] = 1.77 x 10^-3 M

pOH = -log[OH-] = -log(1.77 x 10^-3) = 2.75

pH = 11.25

Part B

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

   = (1.77 x 10^-3 / 0.175 ) x 100

% ionization = 1.01 %

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