Question

The dissociation constant $\left(K_a\right)$ and percent of degree of dissociation $\left(\alpha \right)$ of a weak monobasic acid solution of $0.1M$ with a $pH=5$ are respectively :

• A. ${10}^{-9},1$
• B. ${10}^{-9},{10}^{-4}$
• C. ${10}^{-9},{10}^{-2}$
• D. ${10}^{-5},{10}^{-2}$

Answer 1

The correct option is C ${10}^{-9},{10}^{-2}$

$HX\to H^{+}+X^{-}$

let the amount dissociated be $x$

so concentration of monobasic acid now is $0.1-x$

concentration of $H^{+}$ formed=$x$

Concentration of $X^{-}$ formed =$x$

$K_a=\frac{\left[H^{+}\right]\left[X^{-}\right]}{\left[HX\right]}$

given $x={10}^{-5}$

so $K_a=\frac{X^2}{0.1}$

$K_a={10}^{-9}$

Dissociation percentage $=\frac{{10}^{-5}}{0.1}\times 100={10}^{-2}$