   # Consider the titration of 100.0 mL of 0.100 M HCN by 0.100 M KOH at 25°C. (K a for HCN = 6.2 × 10 −10 .) a. Calculate the pH after 0.0 mL of KOH has been added. b. Calculate the pH after 50.0 mL of KOH has been added. c. Calculate the pH after 75.0 mL of KOH has been added. d. Calculate the pH at the equivalence point. e. Calculate the pH after 125 mL of KOH has been added. ### 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 105CWP
Textbook Problem
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## Consider the titration of 100.0 mL of 0.100 M HCN by 0.100 M KOH at 25°C. (Ka for HCN = 6.2 × 10−10.)a. Calculate the pH after 0.0 mL of KOH has been added.b. Calculate the pH after 50.0 mL of KOH has been added.c. Calculate the pH after 75.0 mL of KOH has been added.d. Calculate the pH at the equivalence point.e. Calculate the pH after 125 mL of KOH has been added.

(a)

Interpretation Introduction

Interpretation: The pH of the solutions after the addition of different volumes of 0.100M KOH are to be calculated.

Concept introduction: HCN is a weak acid and KOH is a strong base. The reaction between an acid and a base takes place with the formation of a salt and a water molecule.

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.

To determine: The pH of the solution after the addition of 0.0mL of KOH .

### Explanation of Solution

Explanation

Given

Titration of 100.0ml of 0.100MHCN with 0.100MKOH .

The acid dissociation constant Ka is 6.2×1010 .

The given solution is the combination of acid and a base. The acid-base reaction for this combination is given as,

HCN+KOHKCN+H2O

When 0.0mlKOH is added to the solution it means only HCN is present in the solution.

Therefore the equilibrium of HCN is represented by the equation,

HCNH++CN-

Molarity of HCN is 0.100M . The number of moles is calculated by the formula,

n=C×V

Where,

• n is the number of moles.
• C is the concentration of the solution.
• V is the volume of the solution.

Substitute the values of concentrations and volume in the above equation.

n=C×V=0.100M×1001000L=0.01mol

Now x is supposed to be the change in moles. The equilibrium reaction with the calculated moles is expressed in ICE (initial, change, equilibrium) table as,

HCNH++CN-Initialmol0.0100Changeinmolx+x+xEquilibriummol0

(b)

Interpretation Introduction

Interpretation: The pH of the solutions after the addition of different volumes of 0.100M KOH are to be calculated.

Concept introduction: HCN is a weak acid and KOH is a strong base. The reaction between an acid and a base takes place with the formation of a salt and a water molecule.

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.

To determine: The pH of the solution after the addition of 50.0mL of KOH .

(c)

Interpretation Introduction

Interpretation: The pH of the solutions after the addition of different volumes of 0.100M KOH are to be calculated.

Concept introduction: HCN is a weak acid and KOH is a strong base. The reaction between an acid and a base takes place with the formation of a salt and a water molecule.

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.

To determine: The pH of the solution after the addition of 75.0mL of KOH .

(d)

Interpretation Introduction

Interpretation: The pH of the solutions after the addition of different volumes of 0.100M KOH are to be calculated.

Concept introduction: HCN is a weak acid and KOH is a strong base. The reaction between an acid and a base takes place with the formation of a salt and a water molecule.

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.

To determine: The pH of the solution at the equivalence point.

(e)

Interpretation Introduction

Interpretation: The pH of the solutions after the addition of different volumes of 0.100M KOH are to be calculated.

Concept introduction: HCN is a weak acid and KOH is a strong base. The reaction between an acid and a base takes place with the formation of a salt and a water molecule.

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.

To determine: The pH of the solution after the addition of 125.0mL of KOH .

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