   Chapter 15, Problem 14Q

Chapter
Section
Textbook Problem

# Consider the following four titrations.i. 100.0 mL of 0.10 M HCl titrated by 0.10 M NaOHii. 100.0 mL of 0.10 M NaOH titrated by 0.10 M HCliii. 100.0 mL of 0.10 M CH3NH2 titrated by 0.10 M HCliv. 100.0 mL of 0.10 M HF titrated by 0.10 M NaOH Rank the titrations in order of:a. increasing volume of titrant added to reach the equivalence point.b. increasing pH initially before any titrant has been added.c. increasing pH at the halfway point in equivalence.d. increasing pH at the equivalence point.How would the rankings change if C5H5N replaced CH3NH2 and if HOC6H5 replaced HF?

(I)

Interpretation Introduction

Interpretation: The four different titrations are given. Various questions based on the given titrations are to be answered.

Concept introduction: Titration is a quantitative chemical analysis method that is used for the determination of concentration of an unknown solution. In acid base titration, the neutralization of either acid or base is done with a base or acid respectively of known concentration. This helps to determine the unknown concentration of acid or base.

When the amount of the titrant added is just sufficient for the neutralization of analyte is called equivalence point. At this point equal equivalents of both the acid and base are added.

To determine: The increasing pH orders for the given points in titration reaction.

Explanation

Explanation

(a)

To determine: The titrations in order of increasing volume of titrant added to reach the equivalence point.

In all the four cases same volume of titrant, that is 100mL is required to reach the equivalence point.

In part (i), the concentration of the strong acid and strong base is same and as the reaction involves the strong acid and strong base, therefore it involves 1:1 ratio. As the concentration of both the acid and base is same, therefore the same volume of NaOH and HCl is required for reaching the equivalence point.

In part (ii), the concentration of the strong acid and strong base is same and as the reaction involves the strong acid and strong base, therefore it involves 1:1 ratio. As the concentration of both the acid and base is same, therefore the same volume of NaOH and HCl is required for reaching the equivalence point.

In part (iii), the concentration of the acid and base is same and as the reaction involves the acid and base, therefore it involves 1:1 ratio. As the concentration of both the acid and base is same, therefore the same volume of CH3NH2 and HCl is required for reaching the equivalence point.

In part (iv), the concentration of the acid and base is same and as the reaction involves the acid and base, therefore it involves 1:1 ratio. As the concentration of both the acid and base is same, therefore the same volume of HF and NaOH is required for reaching the equivalence point.

Therefore, in all the four cases same volume of titrant, that is 100mL is required to reach the equivalence point.

Explanation

(b)

To determine: The titrations in order of increasing pH initially before any titrant has been added.

The order of increasing pH before any titrant is initially added is (i)<(iv)<(ii)<(iii) .

In part (i), the solution initially contains HCl , that is a strong acid and undergoes complete dissociation and forms H+ and Cl . Therefore, the value of pH is lower than 4 .

In part (ii), the solution initially contains NaOH , that is a strong base and it undergoes complete dissociation and forms Na+ and OH . Due to the presence of Hydroxide ions the value of pH is higher and therefore, the value of pH is greater than 12 .

In part (iii), the solution initially contains CH3NH2 , that is a weak base and it does not undergo complete dissociation and therefore, the value of pH is greater than 7 but lower than 12 .

In part (iv), the solution initially contains HF , that is a weak acid and it does not undergo complete dissociation and therefore, the value of pH is lower than 7 but greater than 4

(II)

Interpretation Introduction

Interpretation: The four different titrations are given. Various questions based on the given titrations are to be answered.

Concept introduction: Titration is a quantitative chemical analysis method that is used for the determination of concentration of an unknown solution. In acid base titration, the neutralization of either acid or base is done with a base or acid respectively of known concentration. This helps to determine the unknown concentration of acid or base.

When the amount of the titrant added is just sufficient for the neutralization of analyte is called equivalence point. At this point equal equivalents of both the acid and base are added.

To determine: The value of pH of solution when 150.0mL KOH has been added to it.

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