Derive the equation of ionization constants ${K_a}$ of weak acids $HX$.
A weak acid $HX$ that is partially ionized in the aqueous solution. The equilibrium can be expressed by :
$\text { Equilibrium : } \text { Concentration M : } \mathrm{HX}_{\text {(aq) }}+\mathrm{H}_{2} \mathrm{O}_{(l)}+\mathrm{H}_{3} \mathrm{O}_{\text {(aq) }}^{+}+\mathrm{X}_{\text {(aq) }}^{-} $
$\text {Change in conce. : } -\mathrm{C} \alpha \ \ 0 \ \ \ 0 $
$ \text { Concentration at } (\mathrm{C}-\mathrm{C} \alpha) (0+\mathrm{C} \alpha) (0+\mathrm{C} \alpha) $
$ \text { Equili. in molarity : } =\mathrm{C}(1-\alpha) \mathrm{C} \alpha \mathrm{C} \alpha $
$ =\mathrm{C} \alpha =\mathrm{C} \alpha$
where, $\alpha=$ Extent upto which $\mathrm{HX}$ is ionized into ions.
Suppose, initial concentration of undissociated acid $\mathrm{HX}=\mathrm{C} \alpha \mathrm{M}$
$\therefore \quad$ Ionized $\mathrm{HX}=\mathrm{C} \alpha \mathrm{M}$
At equilibrium $[\mathrm{HX}]=(\mathrm{C}-\mathrm{C} \alpha)=\mathrm{C}(1-\alpha)$
$\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]=\left[\mathrm{X}^{-}\right]=\mathrm{C} \alpha \mathrm{M}$
The equilibrium constant for the above discussed acid-dissociation equilibrium:
$\mathrm{K}=\frac{\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\left[\mathrm{X}^{-}\right]}{[\mathrm{HX}]\left[\mathrm{H}_{2} \mathrm{O}\right]} \quad \therefore \mathrm{K}\left[\mathrm{H}_{2} \mathrm{O}\right]=\frac{\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\left[\mathrm{X}^{-}\right]}{[\mathrm{HX}]}$
In $\left[\mathrm{H}_{2} \mathrm{O}\right]=$ constant $=\mathrm{K}^{\prime}$ So,
$\quad \mathrm{K}\left[\mathrm{H}_{2} \mathrm{O}\right]=\mathrm{K} \times \mathrm{K}^{\prime}$ new constant $\mathrm{K}_{a}$
where, $\mathrm{K}_{a}=$ Ionization constant of weak acid.
$\therefore \mathrm{K}_{a}=\frac{\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\left[\mathrm{X}^{-}\right]}{[\mathrm{HX}]}=\frac{\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]^{2}}{[\mathrm{HX}]} \ldots .($ Eq. $-{i})$
$\therefore \mathrm{K}_{a}=\frac{(\mathrm{C} \alpha)(\mathrm{C} \alpha)}{\mathrm{C}(1-\alpha)}=\frac{\mathrm{C} \alpha^{2}}{\left(1-\alpha^{2}\right)} \ldots . .( Eq.-ii )$
Above equation-$(ii)$ is used for calculation of ionization constant of weak acid $HX$.
In aqueous solution the ionization constants for carbonic acid are
$K_1 = 4.2 \times 10^{-7}$ and $K_2 = 4.8 \times 10^{-11}$
Select the correct statement for a saturated $0.034\, M$ solution of the carbonic acid.
Write characteristics and uses of ${K_a}$ value.
For a weak acid, the incorrect statement is
Calculate the degree of ionization of $0.05 \,M$ acetic acid if its $p K_{ a }$ value is $4.74$
How is the degree of dissociation affected when its solution also contains $(a)$ $0.01 \,M$ $(b)$ $0.1 \,M$ in $HCl$ ?
${K_{C{H_3}COOH}} = 1.9 \times {10^{ - 5}}$. Calculate $pH$ at end point in titration of $0.1$ $M$ $C{H_3}COOH$ and $0.1$ $M$ $NaOH$.