   # A 0.15- M solution of a weak acid is 3.0% dissociated. Calculate K a . ### 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 13, Problem 75E
Textbook Problem
1 views

## A 0.15-M solution of a weak acid is 3.0% dissociated. Calculate Ka.

Interpretation Introduction

Interpretation: The Ka value for the given solution of a weak acid is to be calculated.

Concept introduction: At equilibrium, the equilibrium constant expression is expressed by the formula,

Ka=ConcentrationofproductsConcentrationofreactants

The percent dissociation of an acid is calculated by the formula,

Percentdissociation=Equilibriumconcentrationof[H+]Initialconcentrationoftheacid×100

### Explanation of Solution

Explanation

To determine: The Ka value for the given solution of a weak acid.

The amount of the weak acid that dissociated is 4.5×10-3M_ .

Given

The percent dissociation is 3% .

The initial concentration of the given weak acid solution is 0.15M .

The percent dissociation of an acid is calculated by the formula,

Percentdissociation=Equilibriumconcentrationof[H+]Initialconcentrationoftheacid×100

Substitute the value of percent dissociation and the initial concentration of the acid solution in the above expression.

3=Equilibriumconcentrationof[H+]0.15×100

Simplify the above expression.

Equilibriumconcentrationof[H+]=3100×0.15=4.5×10-3M_

The equilibrium constant expression for the given reaction is, Ka=[H+][A][HA]

The weak acid is assumed to be HA .

The dominant equilibrium reaction for the given case is,

HA(aq)H+(aq)+A(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+][A][HA] (1)

The equilibrium constant expression for the given reaction is, Ka=[x]2[0.15]

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

The ICE table for the stated reaction is,

HA(aq)H+(aq)+A(aq)Inititialconcentration0

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