   Chapter 14, Problem 81E

Chapter
Section
Textbook Problem

# One mole of a weak acid HA was dissolved in 2.0 L of solution. After the system had come to equilibrium, the concentration of HA was found to be 0.45 M. Calculate Ka for HA.

Interpretation Introduction

Interpretation: A solution formed by dissolving one mole of a weak acid HA in 2.0L of solution and the equilibrium concentration of HA is given. The value of Ka for the given weak 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 value of Ka for the given weak acid.

The initial concentration of the weak acid is 0.5M_ .

Given

Moles of the acid taken is 1mol .

The volume of the solution is 2.0L .

The concentration of a substance is calculated by the formula,

Concentration=NumberofmolesofthesubstanceVolume(L)

Substitute the given value of the number of moles of the acid and the volume of the solution in the above expression.

InitialConcentrationofacid=1mol2L=0.5M_

The equilibrium constant expression for the dissociation reaction of the weak acid HA is,

Ka=[H+][A][HA]

HA is given to be a weak acid. Hence, it does not completely dissociate in water.

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 [H+]  and [A] is 0.05M_ .

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

The initial concentration of the given weak acid is 0.5M .

The ICE table for the stated reaction is,

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

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