   Chapter 15, Problem 65E

Chapter
Section
Textbook Problem

# Calculate the pH at the halfway point and at the equivalence point for each of the following titrations.a. 100.0 mL of 0.10 M HC7H5O2 (Ka = 6.4 × 10−5) titrated by 0.10 M NaOHb. 100.0 mL of 0.10 M C2H5NH2 (Kb = 5.6 × 10−4) titrated by 0.20 M HNO3c. 100.0 mL of 0.50 M HCl titrated by 0.25 M NaOH

(a)

Interpretation Introduction

Interpretation: The halfway point and the equivalence point pH for each of the given titrations are to be calculated.

Concept introduction: The pH value is defined as a figure that is used to express the acidity or basicity of a solution by using the logarithmic scale on which 7 is a neutral. The values lower than 7 shows acidic or greater than 7 shows basic behavior of the solution.

To determine: The halfway point and the equivalence point pH for the given titration.

Explanation

Explanation

Given

Concentration of HC7H5O2 is 0.10M .

Volume of the solution is 100mL .

Concentration of NaOH is 0.10M

According to the half-way titration, the volume of the base consumed is half of the equivalence point. Volume of the base is calculated by using the expression,

M1V1=M2V2

Where,

• M1 is the concentration of HC7H5O2 .
• M2 is the concentration of NaOH .
• V1 is the volume of HC7H5O2 solution.
• V2 is the volume of NaOH solution.

Substitute the values of M1 , V1 , and M2 in the above expression to calculate the volume of NaOH solution.

M1V1=M2V20.10M×100mL=0.10M×V2V2=100mL

The volume of the base at the equivalence point is V2=100mL .

The volume of the base at the half-way point is 50mL .

The reaction between C6H5COOH and NaOH at the half-way equivalence point is,

C6H5COOH+NaOHC6H5COONa+H2O0.01mole0.005mole0.005mole

The pH at half-way point is calculated by using the formula,

pH=pKa+log[Salt][Acid]

The pKa of benzoic acid is 4.20 .

Substitute the values of pKa and concentration of salt and acid in the above formula to calculate the pH at half-way point.

pH=4.20+log[0.005][0.005]=4.20_

Hence, the pH at the half-way equivalence point is 4.20_ .

At equivalence point, the reaction between C6H5COOH and NaOH is,

C6H5COOH+NaOHC6H5COONa+H2O0

(b)

Interpretation Introduction

Interpretation: The halfway point and the equivalence point pH for each of the given titrations are to be calculated.

Concept introduction: The pH value is defined as a figure that is used to express the acidity or basicity of a solution by using the logarithmic scale on which 7 is a neutral. The values lower than 7 shows acidic or greater than 7 shows basic behavior of the solution.

To determine: The halfway point and the equivalence point pH for the given titration.

(c)

Interpretation Introduction

Interpretation: The halfway point and the equivalence point pH for each of the given titrations are to be calculated.

Concept introduction: The pH value is defined as a figure that is used to express the acidity or basicity of a solution by using the logarithmic scale on which 7 is a neutral. The values lower than 7 shows acidic or greater than 7 shows basic behavior of the solution.

To determine: The halfway point and the equivalence point pH for the given titration.

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