Chapter 23, Problem 88A

Interpretation Introduction

Interpretation: To find the increasing order of the acidic strength based on the given ${\text{K}}_{\text{a}}$ values.

Concept Introduction: When the ${\text{K}}_{\text{a}}$ value is low then the dissociation will be very less. It means that the acid will have a higher ${\text{pK}}_{\text{a}}$ value. Hence an acid with less ${\text{K}}_{\text{a}}$ value is said to be a weak acid.

Answer to Problem 88A

  $3.5\times {10}^{-6}<1.5\times {10}^{-5}<6.6\times {10}^{-5}<2.7\times {10}^{-3}$

Explanation of Solution

When the ${\text{pK}}_{\text{a}}$ values for the given ${\text{K}}_{\text{a}}$ values are calculated then the values will be as follows:

For ${\text{K}}_{\text{a}}$ value of $3.5\times {10}^{-6}$ , the ${\text{pK}}_{\text{a}}$ value will be 5.45

For ${\text{K}}_{\text{a}}$ value of $2.7\times {10}^{-3}$ , the ${\text{pK}}_{\text{a}}$ value will be 2.56

For ${\text{K}}_{\text{a}}$ value of $1.5\times {10}^{-5}$ , the ${\text{pK}}_{\text{a}}$ value will be 4.82

For ${\text{K}}_{\text{a}}$ value of $6.6\times {10}^{-5}$ , the ${\text{pK}}_{\text{a}}$ value will be 4.18

From the ${\text{pK}}_{\text{a}}$ it is clear that higher ${\text{pK}}_{\text{a}}$ will decrease the acidic strength. Thus, the increasing order is justified.

Conclusion

When an acid dissociate completely it is said to be a strong acid and when the acid does not dissociate completely it is said to be a weak acid. This strength of the acid can be determined by knowing the ${\text{K}}_{\text{a}}$ values.