   Chapter 14, Problem 69E

Chapter
Section
Textbook Problem

# Monochloroacetic acid, HC2H2ClO2, is a skin irritant that is used in “chemical peels” intended to remove the top layer of dead skin from the face and ultimately improve the complexion. The value of Ka for monochloroacetic acid is 1.35 × 10−3. Calculate the pH of a 0.10-M solution of monochloroacetic acid.

Interpretation Introduction

Interpretation: The pH of the given 0.10M solution of monochloroacetic acid, with the given value of Ka , 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. A logarithmic scale is used on which, the value 7 corresponds to a neutral species, a value less than 7 corresponds to an acid and a value greater than 7 corresponds to a base.

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 pH of the given 0.10M solution of monochloroacetic acid.

The equilibrium constant expression for the stated dissociation reaction is,

Ka=[H+][C2H2ClO2][HC2H2ClO2]

The dominant equilibrium reaction for the given case is,

HC2H2ClO2(aq)H+(aq)+C2H2ClO2(aq)

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

Ka=ConcentrationofproductsConcentrationofreactants

Where,

• Ka is the acid dissociation constant.

The equilibrium constant expression for the given reaction is,

Ka=[H+][C2H2ClO2][HC2H2ClO2] (1)

The [H+] is 1.2×10-2M_ .

The change in concentration of HC2H2ClO2 is assumed to be x .

The ICE table for the stated reaction is,

HC2H2ClO2(aq)H+(aq)+C2H2ClO2(aq)Inititialconcentration0.1000Changex+x+xEquilibriumconcentration0.10xxx

The equilibrium concentration of [HC2H2ClO2] is (0.10x)M .

The equilibrium concentration of [H+] is xM

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