Chapter 16, Problem 16.158MP

Interpretation Introduction

(a)

Interpretation:

Thevalue of $\text{pH}$, $\text{50mL HF}$ solution needs to be determined.

Concept introduction:

For a weak acid say HA, the dissociation reaction is represented as follows:

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

The expression for acid dissociation constant can be represented as follows:

$K_a=\frac{\left[{\text{H}}^{+}\right]\left[{\text{A}}^{-}\right]}{\left[\text{HA}\right]}$

Here, $\left[{\text{H}}^{+}\right]$, $\left[{\text{A}}^{-}\right]$ and $\left[\text{HA}\right]$ are equilibrium concentrations of hydrogen ion, concentration of conjugate base of HA and concentration of weak acid HA respectively.

The pH of the solution can be calculated as follows:

$\text{pH}=-\log \left[{\text{H}}^{+}\right]$

Here, $\left[{\text{H}}^{+}\right]$ is the concentration of hydrogen ion in the solution.

Interpretation Introduction

(b)

Interpretation:

The volume of a solution to triple the percent dissociation needs to be determined by explaining the procedure of solving the problem.

Concept introduction:

The percent dissociation is directly proportional to the square root of the value of acid-dissociation constant and inversely proportional to the square root of the initial concentration of hydrofluoric acid $\text{HF}$

$\text{percent dissociation }\propto \sqrt{\frac{{\text{K}}_{\text{a}}}{\left[\text{HF}\right]}}$

Here, K_a is acid dissociation constant and $\left[\text{HF}\right]$ is the concentration of hydrogen fluoride acid.