Question

The dissociation constant of a weak acid, $HA$ is $4.9\times {10}^{-8}$. Then $pH$ of its decinormal solution will be near to:

• A. $3.5$
• B. $4.15$
• C. $5.3$
• D. $2.5$

Answer 1

Answer: (B)

$4.15$

Decinormal solution has normality $0.1$ N.

The relationship between the dissociation constant of a weak acid and hydrogen ion concentration is, $[H{]}^{+}=\sqrt{K_a\times c}$.

Substitute values in the above equation.
$[H{]}^{+}=\sqrt{4.9\times {10}^{-8}\times 0.1}=7\times {10}^{-5}$.

pH $=-log[H{]}^{+}=-log(7\times {10}^{-5})$

$=5-log7$

$=4.15$.

Option B is correct.