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Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640

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Section
BuyFindarrow_forward

Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640
Chapter 17, Problem 3PS
Textbook Problem
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What is the pH of a solution that consists of 0.20 M ammonia, NH3, and 0.20 M ammonium chloride, NH4Cl?

Interpretation Introduction

Interpretation:

The pHof solution containing 0.2M ammonia and 0.2M ammonium chloride has to be determined.

Concept introduction:

In aqueous solution an base undergoes ionization. The ionization of base is expressed in terms of the equilibrium constant. The quantitative measurement tells about the strength of the base. Higher the value of Kbstronger will be the base. The base dissociation can be represented as following equilibrium,

  Baq+H2OlBH+aq+OHaq

A weak base undergoes partial dissociation in aqueous solution. The expression for base dissociation constant Kb is given as,

  Kb=BH+eqOHeqBeq                                                                (1)

Here,

  BH+eq is the equilibrium concentration of conjugate acid of the base.

  OHeq is the equilibrium concentration of hydroxide ion.

  Beq is the equilibrium concentration of base.

The ICE table (1) gives the relationship between the concentrations of species at equilibrium.

EquationBaq+H2OaqBH+aq+OHInitialMc00ChangeMx+x+xEquilibriumMcxxx

From the ICE table (1),

  BH+eq=xOHeq=xBeq=cx

Substitute x for BH+eq, x for OHeqand cx for Beqin equation (1).

The base dissociation constant will be

  Kb=xxcx=x2cx

  Kb=x2cx                                                                           (2)

This table can be modified if one of the ion is already present before the ionization of acid. Then there will be some extent of suppression of dissociation of the weak base. This can be explained on the basis of Le-Chatelier’s principle. According to which reaction will be more on the left side rather than right if one the ion from product side is already present before equilibrium. This suppression of ionization of weak base in presence of strong electrolyte having common ion is called as “Common Ion effect”.

Therefore a modified ICE table is used to give the concentration relationships between ions. For example, if cation BH+ is already present in the solution before equilibrium,

EquationBaq+H2OaqBH+aq+OHInitialMcy0ChangeMx+x+xEquilibriumMcxy+xx

Here,

y is the initial concentration of the cation BH+(common ion coming from strong electrolyte) present in the solution before the dissociation of a weak base HA.

From the ICE table (2),

  H3O+eq=xAeq=x+yHAeq=cx

Substitute x forH3O+eq, x+y for Aeqand cx for HAeqin (1).

The expression for the base dissociation constant, Kb, will be given as,

  Kb=xx+ycx

There is an assumption for common ion effect, according to which the value of “x” is very small on comparing to the initial concentration of baseBc”and initial concentration of cationBH+, “y”. Thus x can be neglected with respect to yand c.

Then Kb can be written as,

  Kb=xyc                                                                                         (3)

The expression for pOH is,

  pOH=logOH                                                                              (4)

The pHof the solution is calculated by using the relation,

  pH+pOH=pKw                                                                                   (5)

The value of pKw is 14.0.

Explanation of Solution

Given data is as follows:

The value of base dissociation constant, Kb, for ammonia is1.8×105.

The initial concentration of ammonia is 0.20M.

The initial concentration of ammonium chloride is0.20M.

Ammonia undergoes dissociation in aqueous solution and the reaction is given as,

    NH3aq+H2OaqNH4+aq+OHaq

Ammonium chloride is a strong electrolyte and dissociates to give ammonium ion and chloride ion in aqueous solution.

    NH4Claq+H2OlNH4+aq+Claq

In presence of ammonium chloride having common ammonium ion, due to the common ion effect suppression of dissociation of ammonia will occur.

The ICE table (3) is given as follows,

EquationNH3aq+H2OaqNH4+aq+OHaqInitialM0.200.200ChangeMx+x+xEquilibriumM0.20x0.20+xx

The value of x i.e., the concentration of hydroxide ion can be calculated by using equation (3).

  Kb=x×yc

Rearrange for x.

  x=Kbcy

Substitute 1

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Chapter 17 Solutions

Chemistry & Chemical Reactivity
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