Chapter 16, Problem 78QP

Interpretation Introduction

Interpretation:

From the given salt solution systems, the ${\text{X}}^{-}$ ion that has the weakest conjugate acidand the decreasing order of the basic strength of the given anions, is to be determined.

Concept introduction:

A salt is a strong electrolyte, which dissociates completely when added to water.

When a salt contains an anion that comes from a weak acid then the anion recombines with water to produce a weak acid and hydroxide ions, and forms a basic solution. The cation, if it comes from a strong base, does not recombine and is present in the solution as a free ion without having any effect on the pH of the solution.

The reaction of the salt $\left(\text{BA}\right)$ which takes place is:

${\text{A}}^{-}{}_{\left(\text{aq}\right)}+{\text{H}}_2{\text{O}}_{\left(\text{l}\right)}\rightleftharpoons {\text{HA}}_{\left(\text{aq}\right)}+{\text{OH}}^{-}{}_{\left(\text{aq}\right)}$

Here, ${\text{A}}^{-}$ comes from the weak acid $\text{HA}$ and ${\text{B}}^{+}$ comes from strong base $\text{BOH}$. The pH of this solution is determined by the $\left[{\text{OH}}^{-}\right]$

The relationship between ${\text{K}}_{\text{b}}$, ${\text{K}}_{\text{a}}$ and ${\text{K}}_{\text{w}}$ gives the quantitative basis of the reciprocal relationship between the strength of an acid and its conjugate base or vice-versa.

${\text{K}}_{\text{a}}\times {\text{K}}_{\text{b}}={\text{K}}_{\text{w}}$ …… (1)

${\text{K}}_{\text{b}}$ is the measure of dissociation of a base and is known as the base-ionization constant, which is specific at a particular temperature.

${\text{K}}_{\text{b}}=\frac{\left[{\text{OH}}^{-}\right]\left[\text{HA}\right]}{\left[{\text{A}}^{-}\right]}$ …… (2)