   Chapter 14, Problem 8RQ

Chapter
Section
Textbook Problem

# For conjugate acid–base pairs, how are Ka and Kb related? Consider the reaction of acetic acid in water CH 3 CO 2 H ( a q ) + H 2 O ( l ) ⇌ CH 3 CO 2 − ( a q ) + H 3 O + ( a q ) where Ka = 1.8 × 10−5a. Which two bases are competing for the proton?b. Which is the stronger base?c. In light of your answer to part b. why do we classify the acetate ion (CH3CO2−) as a weak base? Use an appropriate reaction to justify your answer.In general, as base strength increases, conjugate acid strength decreases. Explain why the conjugate acid of the weak base NH3 is a weak acid.To summarize, the conjugate base of a weak acid is a weak base and the conjugate acid of a weak base is a weak acid (weak gives you weak). Assuming Ka for a monoprotic strong acid is 1 × 106, calculate Kb for the conjugate base of this strong acid. Why do conjugate bases of strong acids have no basic properties in water? List the conjugate bases of the six common strong acids. To tie it all together, some instructors have students think of Li+, K+, Rb+, Cs+, Ca2+, Sr2+, and Ba2+ as the conjugate acids of the strong bases LiOH, KOH. RbOH, CsOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. Although not technically correct, the conjugate acid strength of these cations is similar to the conjugate base strength of the strong acids. That is, these cations have no acidic properties in water; similarly, the conjugate bases of strong acids have no basic properties (strong gives you worthless). Fill in the blanks with the correct response. The conjugate base of a weak acid is a_____base. The conjugate acid of a weak base is a_____acid. The conjugate base of a strong acid is a_____base. The conjugate acid of a strong base is a_____ acid. (Hint: Weak gives you weak and strong gives you worthless.)

(I)

Interpretation Introduction

Interpretation: The given questions based upon conjugate acid-base pairs are to be answered.

Concept introduction: The conjugate base of a weak acid is a strong base. The conjugate acid of a weak base is a strong acid. The conjugate base of a strong acid is a weak base. The conjugate acid of a strong base is a weak acid.

Explanation

Explanation

To determine: The relationship between Ka and Kb .

For conjugate acid-base pairs, the Ka and Kb are related by the following equation

(II)  (a)

Interpretation Introduction

Interpretation: The given questions based upon conjugate acid-base pairs are to be answered.

Concept introduction: The conjugate base of a weak acid is a strong base. The conjugate acid of a weak base is a strong acid. The conjugate base of a strong acid is a weak base. The conjugate acid of a strong base is a weak acid.

To determine: The two bases in the given reaction that are competing for the protons.

(b)

Interpretation Introduction

Interpretation: The given questions based upon conjugate acid-base pairs are to be answered.

Concept introduction: The conjugate base of a weak acid is a strong base. The conjugate acid of a weak base is a strong acid. The conjugate base of a strong acid is a weak base. The conjugate acid of a strong base is a weak acid.

To determine: The stronger base.

(c)

Interpretation Introduction

Interpretation: The given questions based upon conjugate acid-base pairs are to be answered.

Concept introduction: The conjugate base of a weak acid is a strong base. The conjugate acid of a weak base is a strong acid. The conjugate base of a strong acid is a weak base. The conjugate acid of a strong base is a weak acid.

To determine: The reason for classifying the acetate ion as a weak base with an appropriate reaction.

(III)

Interpretation Introduction

Interpretation: The given questions based upon conjugate acid-base pairs are to be answered.

Concept introduction: The conjugate base of a weak acid is a strong base. The conjugate acid of a weak base is a strong acid. The conjugate base of a strong acid is a weak base. The conjugate acid of a strong base is a weak acid.

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