   Chapter 15, Problem 120IP

Chapter
Section
Textbook Problem

# A 10.00-g sample of the ionic compound NaA, where A− is the anion of a weak acid, was dissolved in enough water to make 100.0 mL of solution and was then titrated with 0.100 M HCl. After 500.0 mL HCl was added, the pH was 5.00. The experimenter found that 1.00 L of 0.100 M HCl was required to reach the stoichiometric point of the titration.a. What is the molar mass of NaA?b. Calculate the pH of the solution at the stoichiometric point of the titration.

(a)

Interpretation Introduction

Interpretation: The titration of NaA with HCl is given. The molar mass of NaA and pH of the solution at stoichiometric point of titration is to be calculated.

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 molar mass of NaA .

Explanation

Explanation

The given mass of NaA is 10.00g .

The volume of given solution of A is 100.0mL .

The pH of solution is 5.0 .

The volume of HCl at pH=5.0 is 500mL .

The volume of HCl at stoichiometric point is 1.0L .

The concentration of HCl at stoichiometric point is 0.100M .

The concentration of any species is given as,

Concentration=NumberofmolesVolumeofsolutioninlitres (1)

Rearrange the above equation to obtain the value of number of moles.

Numberofmoles=Concentration×Volumeofsolutioninlitres (2)

Substitute the value of concentration and volume of HCl to obtain the number of moles present at the stoichiometric point,

Numberofmoles=Concentration×Volumeofsolutioninlitres=0.100M×1.00L=0

(b)

Interpretation Introduction

Interpretation: The titration of NaA with HCl is given. The molar mass of NaA and pH of the solution at stoichiometric point of titration is to be calculated.

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 pH of the solution at stoichiometric point of titration.

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