   Chapter 16, Problem 56PS

Chapter
Section
Textbook Problem

A hypothetical weak base has K b = 5 .0×10 − 4 .Calculate the equilibrium concentrations of the base, its conjugate acid, and OH- in a 0 .15M solution of the base.

Interpretation Introduction

Interpretation:

The equilibrium concentrations of the base, its conjugate acid, and OH- in a 0.15M solution of the base has to be calculated.

Concept introduction:

Equilibrium constants:

The equilibrium constant is used to quantitative measurement of the strength of the acid and bases in the water.

Ka is acid constant for equilibrium reactions.

HA + H2OH3O++ A-Ka[H3O+][A-][HA]

Kb is base constant for equilibrium reaction.

BOH + H2OB++ OH-Ka[B+][OH-][BOH]

Explanation

The hypothetical base reacts with water. The equilibrium reaction is as follows.

B(aq) + H2O (l) BH+(aq) + OH-(aq)

The equilibrium expression,

Enter the ICE table the concentrations before equilibrium is established, the change that occurs as the reaction proceeds to equilibrium and the concentrations when equilibrium has been achieved.

B(aq) + H2O (l) BH+(aq) + OH-(aq)I         0.15                --                --                   --C          -x                 --              + x                  +xE     (0.15 - x)          --                 x                    x

Kb of weak base is 5.0 × 10-4

At equilibrium, x = [OH-] [BH+]

Kb(Base) = [BH+][OH-][B]

5

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