Problem: A 0.140 M solution of a weak base has a pH of 11.25.Determine Kb for the base.

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We’re asked to determine the Kb for a 0.140 M solution of a weak base with a pH of 11.25.

Recall that the Equilibrium reaction of any weak base, B in water is given by: 

B(aq) + H2O(l)  ⇌  HB+(aq) + OH-(aq)

  (reactants)              (products)

Bweak baseproton acceptor
H2O → will act as the weak acidproton donor

We write the Kb expression for a weak base, B as: 


Since we’re dealing with a weak base and Kb is an equilibrium expression we will have to create an ICE chart to determine the equilibrium concentration of each species:

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Problem Details

A 0.140 M solution of a weak base has a pH of 11.25.

Determine Kb for the base.

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