   Chapter 16, Problem 103GQ

Chapter
Section
Textbook Problem

Oxalic acid is a relatively weak diprotic acid. Calculate the equilibrium constant for the reaction shown below from Ka1 and Ka2. (See Appendix H for the required Ka values.) H 2 C 2 O 4 (aq) + 2H 2 O( l )  ⇄  C 2 O 4 2 − (aq) + 2H 3 O + (aq)

Interpretation Introduction

Interpretation:

The equilibrium constant has to be calculated for the following rection.

H2C2O4(aq) + 2H2OC2O42-(aq) + 2H3O+(aq)

Concept introduction:

Equilibrium constants:

The equilibrium constant is used to quantitative measurement of the strength of the acid and bases in the water.

Ka is an acid constant for equilibrium reactions.

HA + H2OH3O++ A-Ka[H3O+][A-][HA]

Kb is base constant for equilibrium reaction.

BOH + H2OB++ OH-Ka[B+][OH-][BOH]

Explanation

The dissociation oxalic acid is s follows.

H2C2O4(aq) + H2O(l) H3O+(aq) + HC2O4-(aq)    Ka1=5.9×10-2

H2C2O4(aq) + H2O(l) H3O+(aq) + C2O42-(aq)    Ka2=6.4×105

Equilibrium expression for the above two reaction is as follows.

Ka1[H3O+][HC2O4-][H2C2O4] (1)

Ka2[H3O+][C2O42-][H2C2O4] (2)

But the given reaction is as follows.

H2C2O4(aq) + 2H2OC2O42-(aq) + 2H3O+(aq)

Let’s write the equilibrium expression for the above reaction.

Knet[C2O42-][H3O+][H2C2O4]

The given reaction of oxalic acid can be obtained by the adding of those two reactions

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