Chemistry: Principles and Reactions
Chemistry: Principles and Reactions
8th Edition
ISBN: 9781305079373
Author: William L. Masterton, Cecile N. Hurley
Publisher: Cengage Learning
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Chapter 22, Problem 26QAP

When aniline, C6H5NH2 ( K b = 7.4 × 10 10 ) , reacts with a strong acid, its conjugate acid, C6H5NH3+, is formed. Calculate the pH of a 0.100 M solution of C6H5NH3+ and compare it with the pH of acetic acid ( K a = 1.86 × 10 5 ) .

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Interpretation Introduction

Interpretation:

The pH of 0.100 M solution of C6H4NH3+ is to be obtained and compared with the pH of acetic acid.

Concept introduction:

The relationship between the concentration of products and reactants at equilibrium for a general reaction:

aA+bBcC+dD

Where A, B, C, and D represents chemical species and a, b, c, and d are the coefficients for balanced reaction.

The equilibrium expression, Kc for reversible reaction is determined by multiplying the concentrations of products together and divided by the concentrations of the reactants. Every concentration term is raised to the power that is equal to the coefficient in the balanced reaction. So, the expression is:

Kc=[C]c[D]d[A]a[B]b

pH scale is a type of measurement of H+ ion concentration in the solution. It is taken as a negative log of H+ ion concentration as:

pH=log1o[H+]

For the equilibrium conditions:

ka×kb=kw=1014

Where, ka is acid dissociation constant, kb is base dissociation constant and kw is ionic product of water whose value is fixed as 10-14.

Answer to Problem 26QAP

The required pH=2.93 which is comparable to the pH of acetic acid pH=2.87.

Explanation of Solution

Given Information:

The aniline C6H4NH2(kb=7.4×1010) reacts with a strong acid. Its conjugate acid is C6H4NH3+.

For, acetic acid CH3COOH(ka=1.86×105).

The conjugate acid of aniline is a weak acid which dissociates as:

C6H4NH3+(aq)+H2OH3O+(aq)+C6H4NH2(aq)

Let’s discuss the main reaction in one direction and creating ICE table as:

C6H4NH3+(aq)+H2OH3O+(aq)+C6H4NH2(aq)I    0.100M                            0                 0C      x                                      x                xE    (0.100Mx)                      x                x

Hence, we have:

The equilibrium constant expression will be:

Ka=[H3O+][C6H4NH2][C6H4NH3+]

Putting the standard value of Ka :

ka=kwkb=10147.4×1010=1.35×105

Now, the equilibrium expression is:

1.35×105=x×x[0.100Mx]0.100x0.100x2=1.35×105×0.100=1.35×106x=1.162×103

Further, pH is calculated as:

pH=log[H+=log[1.162×103]=2.93

Further the pH of acetic acid is calculated by its dissociation as:

CH3COOH(aq)+H2O(l)H3O+(aq)+CH3COO(aq)I    0.100M.                               0                 0C      x                                      x                xE    (0.100Mx)                      x                x

Hence, we have:

The equilibrium constant expression will be:

Ka=[H3O+][CH3COO][CH3COOH]

Putting the standard value of Ka the equilibrium expression is:

1.8×105=x×x[0.100Mx]0.100x0.100x2=1.8×105×0.100=1.8×106x=1.34×103

Further, pH is calculated as:

pH=log[H+=log[1.34×103]=2.87

Conclusion

The required pH=2.93 which is comparable to the pH of acetic acid pH=2.87.

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Chapter 22 Solutions

Chemistry: Principles and Reactions

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