Chapter 16, Problem 96GQ

### 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

# Pyridine is weak organic base and readily forms a salt with hydrochloric acid. C 5 H 5 N(aq) + HCl(aq)  →  C 5 H 5 NH + (aq) + Cl − (aq)    Pyridine                          Pyridinium ion What is the pH of a 0.025 M solution of pyridinium hydrochloride [C 5 H 5 NH + ]Cl − ?

Interpretation Introduction

Interpretation:

The pH of 0.025M solution of pyridinium hydrochloride 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 a 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 Ka of C5H5NH+

The KbÂ ofÂ 1.5Ã—Â 10-9

KaÃ—KbÂ =Â KwKa=Â KwKbKw=Â 1.0Ã—10-14KbÂ =Â 1.5Ã—10-9

Substitute the values,

Ka=Â 1.0Ã—10-141.5Ã—10-9Â Â Â Â Â =Â 6.7Ã—10-6

The equilibrium chemical reaction is as follows.

C5H5NH+(aq)Â +Â H2Oâ‡ŒH3O+(aq)Â +Â C5H5N(aq)

The equilibrium expression:

Ka=Â [C5H5N][H3O+][C5H5NH+]

The initial concentration of pyridinium ion is 0.025M.

Â Â Â Â Â Â C5H5NH+(aq)Â +Â H2O(l)Â â‡ŒH3O+(aq)Â +Â C5H5N(aq)IÂ Â Â Â Â Â Â Â Â 0.025MÂ 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 xÂ Â 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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