   # Consider the following four titrations (i–iv): i. 150 mL of 0.2 M NH 3 (K b = 1.8 × 10 −5 ) by 0.2 M HCl ii. 150 mL of 0.2 M HCl by 0.2 M NaOH iii. 150 mL of 0.2 M HOCl (K a = 3.5 × 10 −8 ) by 0.2 M NaOH iv. 150 mL of0.2 M HF (K a = 7.2 × 10 −4 ) by 0.2 M NaOH a. Rank the four titrations in order of increasing pH at the halfway point to equivalence (lowest to highest pH). b. Rank the four titrations in order of increasing pH at the equivalence point. c. Which titration requires the largest volume of titrant (HCl or NaOH) to reach the equivalence point? ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243

#### Solutions

Chapter
Section ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243
Chapter 14, Problem 107CWP
Textbook Problem
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## Consider the following four titrations (i–iv):i. 150 mL of 0.2 M NH3 (Kb = 1.8 × 10−5) by 0.2 M HClii. 150 mL of 0.2 M HCl by 0.2 M NaOHiii. 150 mL of 0.2 M HOCl (Ka = 3.5 × 10−8) by 0.2 M NaOHiv. 150 mL of0.2 M HF (Ka = 7.2 × 10−4) by 0.2 M NaOHa. Rank the four titrations in order of increasing pH at the halfway point to equivalence (lowest to highest pH).b. Rank the four titrations in order of increasing pH at the equivalence point.c. Which titration requires the largest volume of titrant (HCl or NaOH) to reach the equivalence point?

(a)

Interpretation Introduction

Interpretation: The four titrations in order of increasing pH at the halfway point to equivalence (lowest to highest) and at the equivalence point are to be ranked. The titration requires the largest volume of titrant HCl or NaOH is to be stated.

Concept introduction: Titration or acid/base titration is a chemical process of neutralization in which a titrant that is the solution of known concentration reacts with a solution of unknown concentration.

The hydrogen ion concentration of the solution is known as pH of the solution.

It is the negative logarithm of Hydrogen ion concentration.

At the equivalence point the moles of the acid and base in the solution are same.

At the half equivalence point the concentration of titrant is just half of the initial amount.

For the given value of dissociation constant the pH=pKa .

To determine: The rank of four titrations in order of increasing pH at the halfway point to equivalence (lowest to highest).

### Explanation of Solution

Explanation

Given

The four titrations are given as,

1. I. 150ml of 0.2MNH3(Kb=1.8×105) by 0.2MHCl .
2. II. 150ml of 0.2MHCl by 0.2MNaOH .
3. III. 150ml of 0.2MHOCl(Ka=3.5×108) by 0.2MNaOH .
4. IV. 150ml of 0.2MHF(Ka=7.2×104) by 0.2MNaOH .

For the first acid-base titration the pH is calculated by the following,

To get the pH the Henderson equation is written as,

pH=pKa+log[Conjugatebase][Weakacid]

In the titration of conjugate base with weak acid at the half equivalence point the concentration of both becomes same therefore, the Henderson equation written as,

pH=pKa+log[Conjugatebase][Weakacid]pH=pKa+log(1)pH=pKa (1)

The pKb of the base is calculated by the formula,

pKb=log(Kb)

Where,

Kb is the base dissociation constant.

Substitute the value of Kb in the above equation.

pKb=log(Kb)pKb=log(1.8×105)pKb=log(1.8)+5pKb=4.74

The value of pKa is calculated by the formula,

pKa+pKb=14

Substitute the calculated value of pKb in the above equation.

pKa+pKb=14pKa=14pKbpKa=144.74pKa=9.26

Substitute the calculated value of pKa in the equation (1).

pH=pKapH=9.26

For the second acid-base titration the pH is calculated as,

For the strong acid-base titration, the pH at the half equivalence point is the concentration of titrant which is just half of the initial amount of the concentration

(b)

Interpretation Introduction

Interpretation: The four titrations in order of increasing pH at the halfway point to equivalence (lowest to highest) and at the equivalence point are to be ranked. The titration requires the largest volume of titrant HCl or NaOH is to be stated.

Concept introduction: Titration or acid/base titration is a chemical process of neutralization in which a titrant that is the solution of known concentration reacts with a solution of unknown concentration.

The hydrogen ion concentration of the solution is known as pH of the solution.

It is the negative logarithm of Hydrogen ion concentration.

At the equivalence point the moles of the acid and base in the solution are same.

At the half equivalence point the concentration of titrant is just half of the initial amount.

For the given value of dissociation constant the pH=pKa .

To determine: The rank of four titrations in order of increasing pH at the equivalence point.

(c)

Interpretation Introduction

Interpretation: The four titrations in order of increasing pH at the halfway point to equivalence (lowest to highest) and at the equivalence point are to be ranked. The titration requires the largest volume of titrant HCl or NaOH is to be stated.

Concept introduction: Titration or acid/base titration is a chemical process of neutralization in which a titrant that is the solution of known concentration reacts with a solution of unknown concentration.

The hydrogen ion concentration of the solution is known as pH of the solution.

It is the negative logarithm of Hydrogen ion concentration.

At the equivalence point the moles of the acid and base in the solution are same.

At the half equivalence point the concentration of titrant is just half of the initial amount.

For the given value of dissociation constant the pH=pKa .

To determine: The titration that requires the largest volume of titrant to reach the equivalence point.

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