Chemistry: The Molecular Science
Chemistry: The Molecular Science
5th Edition
ISBN: 9781285199047
Author: John W. Moore, Conrad L. Stanitski
Publisher: Cengage Learning
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Chapter 14, Problem 129QRT
Interpretation Introduction

Interpretation:

The acids HM,HQandHZ have to be arranged in order of decreasing strength.  The Ka values of these acids have to be determined where possible.

Concept Introduction:

The conjugate base of a strong acid is weak.  The stronger base has the larger pH.

Expert Solution & Answer
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Answer to Problem 129QRT

The decreasing order of strength of acids HM,HQandHZ is HM>HQ>HZ.  The Ka values of these acids HM,HQandHZ are 1×10-1,1×10-3and1×10-5 respectively.

Explanation of Solution

Given that, the pH values of sodium salts NaM,NaQandNaZ are 7.0,8.0and9.0 respectively.  The decreasing order of pH values of these salts is given below.

  NaM>NaQ>NaZ

Thus, the decreasing order of the acidic strength of the conjugate acids HM,HQandHZ is given below.

  HM>HQ>HZ

The general equation for the equilibrium can be written as given below.

  A(aq)+H2O(l)HA(aq)+OH(aq)

Where, A=M,QorZ

The basic ionization constant for the above equation can be written as given below.

  Kb=[HA][OH][A]

A table can be set up as given below.

A(aq)HA(aq)OH(aq)Initialconc.(M)0.101.0×107Changeinconc.(M)x+x+xEquilibriumconc.(M)0.1xx107+x

The concentration of OH comes from its autoionization that is 1.0×107.

By substituting all the values in the above equation, the value of x can be calculated.

  Kb=(x)(107+x)(0.1x)

  1×107+x=[OH]=10pH=10(14.00pH)=10(pH14.00)x=10(pH14.00)1×107

For NaZ, pH=9.0

  x=10(9.014.00)1×107=1×1051×107=1×105Kb=(1×105)(1×107+1×105)(0.11×105)=1×109

For NaQ, pH=8.0

  x=10(8.014.00)1×107=1×1061×107=1×106Kb=(1×106)(1×107+1×106)(0.11×106)=1×1011

For NaM, pH=7.0

  x=10(7.014.00)1×107=1×1071×107=0

This suggests that the reaction with water does not go toward products at all.  That means KbofM may be so small that the pH of the solution is accounted for exclusively by the ionization of water.  If indeed the base does provide all the OH ions to make the solution’s pH, then [HM]=[OH]=107.0=1×107:

  Kb=(1×107)(1×107)(0.1)=1×1013

For HZ, the value of Ka can be calculated as shown below.

  Ka=KwKb=1×10141×109=1×105

Similarly, for HQ

  Ka=KwKb=1×10141×1011=1×103

For HM

  Ka=KwKb=1×10141×1013=1×101

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Chapter 14 Solutions

Chemistry: The Molecular Science

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