Chapter 16, Problem 98GQ

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# Chloroacetic acid, ClCH 2 CO 2 H , is a moderately weak acid ( K a = 1 .40×10 − 3 ) . If you dissolve 94.5 mg of the acid in water to give 125 ml of solution, what is the pH of the solution?

Interpretation Introduction

Interpretation:

The pH of the chloroacetic acid solution has to be determined.

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 an 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]

Ion product constant for water

Kw= [H3O+][OH-]       =1.00×10-14 pH = -log[H3O+]pOH= -log[OH-]

Relation between pH and pOH

pH + pOH =14

Explanation

Letâ€™s calculate the molarity of chloroacetic acid.

TheÂ givenÂ massÂ ofÂ chloroaceticacidÂ =Â 94.5mgconvertÂ mgÂ intoÂ gramsÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â =Â 94.5mgÂ Ã—Â 1g1000mgÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â =Â 0.0945g

And,

TheÂ givenÂ volumeÂ ofÂ solutionÂ =Â 125mLÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â =125mlÃ—1L1000mLÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â =Â 0.125L

ConcentrationÂ ofÂ chloroceticcidÂ =molesÂ ofÂ chloroaceticacidVolumeÂ ofÂ solutionÂ inÂ LitresÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â =[0.0945gÂ ClCH2CO2H94.5g/molÂ ClCH2CO2H]0.125LÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â =0.008M

The equilibrium chemical reaction of chloroacetic acid is as follows.

ClCH2CO2H(aq)Â +Â H2O(l)â‡ŒClCH2CO2-(aq)Â +Â H3O+(aq)

Equilibrium expression:

Ka=Â [ClCH2CO2-][H3O+][ClCH2CO2H]

The initial concentration =0.008

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.

Â Â Â Â Â Â ClCH2CO2H(aq)Â +Â H2O(l)â‡ŒClCH2CO2-(aq)Â +Â H3O+(aq)IÂ Â Â Â Â Â Â Â Â 0.008MÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â --Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â --Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â --CÂ Â Â Â Â Â Â Â Â Â Â -xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â --Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â +Â xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â +Â xEÂ Â Â Â Â Â Â Â (0

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