Chapter 16, Problem 104QP

Interpretation Introduction

Interpretation:

The pH of the solution that contains the salt sodium fluoride is to be determined.

Concept introduction:

Salt is a strong electrolyte that dissociates completely when added to water.

When a salt contains an anion that comes from a weak acid, the anion recombines with water to produce a weak acid and hydroxide ions, and forms a basic solution. The cation that 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 $\left(\text{BA}\right)$ that takes place is:

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

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

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

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

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

The formula to calculate the pOH of the solution from the concentration of hydroxide ions is:

$\text{pOH}=-\log \left[{\text{OH}}^{-}\right]$ …… (3)

pH is the measure of acidity of a solution that depends on the concentration of hydronium ions and temperature of the solution. The relationship between pH and pOH is:

$\text{pH}+\text{pOH}=14$ …… (4)