   Chapter 14, Problem 77E

Chapter
Section
Textbook Problem

# Trichloroacetic acid (CCl3CO2H) is a corrosive acid that is used to precipitate proteins. The pH of a 0.050-M solution of trichloroacetic acid is the same as the pH of a 0.040-M HClO4 solution. Calculate Ka for trichloroacetic acid.

Interpretation Introduction

Interpretation: The pH of a 0.050M solution of trichloroacetic acid (CCl3CO2H) is given to be the same as the pH of a 0.040M solution of HClO4 . The Ka value for trichloroacetic acid is to be calculated.

Concept introduction: The pH of a solution is define as a figure that expresses the acidity of the alkalinity of a given solution.

The pH of a solution is calculated by the formula, pH=log[H+]

At equilibrium, the equilibrium constant expression is expressed by the formula,

Ka=ConcentrationofproductsConcentrationofreactants

Explanation

Explanation

To determine: The Ka value for trichloroacetic acid.

The pH of the given concentration of HClO4 is 1.40_ .

Given

The concentration of HClO4 is 0.040M .

HClO4 is a strong acid. It completely dissociates in water.

Therefore, the [H+] is equal to the concentration of the acid.

Hence, the [H+] is 0.040M .

The pH of a solution is calculated by the formula,

pH=log[H+]

Substitute the value of [H+] in the above expression.

pH=log[0.040]=1.40_

The pH of the given concentration of trichloroacetic acid is 1.40_ .

Given

The pH of trichloroacetic acid (CCl3CO2H) is the same as the pH of of HClO4

Therefore, the pH of the given concentration of trichloroacetic acid is 1.40_ .

The [H+] for trichloroacetic acid is 0.0398M_ .

The pH of the given concentration of trichloroacetic acid is 1.40_ .

The pH of a solution is calculated by the formula,

pH=log[H+]

Rearrange the above expression to obtain the value of [H+] .

[H+]=10pH

Substitute the pH value of trichloroacetic acid in the above expression.

[H+]=101.40=0.0398M_

The equilibrium constant expression for the dissociation reaction of trichloroacetic acid is, Ka=[H+][CCl3CO2][CCl3CO2H]

C6H5COOH is a comparatively stronger acid than H2O .

The dominant equilibrium reaction for the given case is,

CCl3CO2H(aq)H+(aq)+CCl3CO2(aq)

At equilibrium, the equilibrium constant expression is expressed by the formula,

Ka=ConcentrationofproductsConcentrationofreactants

Where,

• Ka is the acid dissociation constant

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