Chemistry Practice Problems Weak Bases Practice Problems Solution: A 0.140 M solution of a weak base has a pH of 11.2...

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

###### Problem

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

Determine Kb for the base.

###### Solution

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:

$\overline{){{\mathbf{K}}}_{{\mathbf{b}}}{\mathbf{=}}\frac{\mathbf{products}}{\mathbf{reactants}}}\phantom{\rule{0ex}{0ex}}{\mathbf{K}}_{\mathbf{b}}\mathbf{=}\frac{\left[{\mathbf{HB}}^{\mathbf{+}}\right]\left[{\mathbf{OH}}^{\mathbf{-}}\right]}{\left[\mathbf{B}\right]}$

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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