   # Two acids, each approximately 0.01 M in concentration, are titrated separately with a strong base. The adds show the following pH values at the equivalence point: HA, pH = 9.5, and HB, pH = 8.5. (a) Which is the stronger acid, HA or HB? (b) Which of the conjugate bases, A − or B − , is the stronger base? ### Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640

#### Solutions

Chapter
Section ### Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640
Chapter 17, Problem 116SCQ
Textbook Problem
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## Two acids, each approximately 0.01 M in concentration, are titrated separately with a strong base. The adds show the following pH values at the equivalence point: HA, pH = 9.5, and HB, pH = 8.5. (a) Which is the stronger acid, HA or HB? (b) Which of the conjugate bases, A− or B−, is the stronger base?

(a)

Interpretation Introduction

Interpretation:

Two acids HA and HB each having equal concentration 0.01 M are titrated by using a strong base. The pH values at equivalence point for HA is 9.5 and for HB is 8.5.

(a) The stronger acid among them has to be stated.

Concept introduction:

A weak acid-strong base type of titration in which weak acid is titrated against a strong base for example, CH3COOH is titrated against NaOH.

For weak acid-strong base titration the pH value can be calculated at various points before and after equivalence point. The equilibrium established during the titration of weak acid HA or HB  with NaOH is represented as,

HA(aq)+ OH(aq)H2O(l)+A(aq)(acid)                                      (conjugate base)

HB(aq)+ OH(aq)H2O(l)+B(aq)(acid)                                      (conjugate base)

At equivalence point all the acid will be neutralized, and there will be only A or B. The OH will be produced due to the hydrolysis of these anions at equivalence point. The hydrolysis equilibrium is represented as,

A(aq)+H2O(l)OH(aq)+HA(aq)

There is formation of buffer due to hydrolysis thus calculation of pH is done by using Henderson-Hasselbalch equation.

pH=pKa+log[conjugatebase][acid] (1)

By using the value of Kb for the anion, concentration of  OH can be calculated. Thus the value of pH is greater than 7 at equivalence point for the weak acid- strong base titrations.

Lower the value of pKa for the acid stronger will be the acid. Stronger acid will have weaker conjugate base as per according to conjugate acid-base concept.

### Explanation of Solution

Explanation to find the stronger acid is given below.

Given:

The given concentration of each acid HA and HB is 0.01 M.

The pH at the equivalence point for acid HA titration with NaOH is 9.5.

For acid HB value of pH at the equivalence point is 8.5.

To find which acid is stronger, their pKa values are calculated by using equation (1).

pH=pKa+log[conjugatebase][acid]

Due to equal concentration of each acid the second term in equation will be equal for both the cases. Thus pH value will be equal to their pKa values

(b)

Interpretation Introduction

Interpretation:

Two acids HA and HB each having equal concentration 0.01 M are titrated by using a strong base. The pH values at equivalence point for HA is 9.5 and for HB is 8.5.

(b) The stronger conjugate base has to be stated.

Concept introduction:

A weak acid-strong base type of titration in which weak acid is titrated against a strong base for example, CH3COOH is titrated against NaOH.

For weak acid-strong base titration the pH value can be calculated at various points before and after equivalence point. The equilibrium established during the titration of weak acid HA or HB  with NaOH is represented as,

HA(aq)+ OH(aq)H2O(l)+A(aq)(acid)                                      (conjugate base)

HB(aq)+ OH(aq)H2O(l)+B(aq)(acid)                                      (conjugate base)

At equivalence point all the acid will be neutralized, and there will be only A or B. The OH will be produced due to the hydrolysis of these anions at equivalence point. The hydrolysis equilibrium is represented as,

A(aq)+H2O(l)OH(aq)+HA(aq)

There is formation of buffer due to hydrolysis thus calculation of pH is done by using Henderson-Hasselbalch equation.

pH=pKa+log[conjugatebase][acid] (1)

By using the value of Kb for the anion, concentration of  OH can be calculated. Thus the value of pH is greater than 7 at equivalence point for the weak acid- strong base titrations.

Lower the value of pKa for the acid stronger will be the acid. Stronger acid will have weaker conjugate base as per according to conjugate acid-base concept.

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