Question

An acid solution of $pH=6$ is diluted $1000$ times, the $pH$ of the final solution becomes

• A. $6.01$
• B. $9$
• C. $3.5$
• D. $6.99$

Answer 1

Answer: (D)

$\because$ pH = 6
$\therefore \left[H^{+}\right]=10^{-6}M$
After dilution $\left[H^{+}\right]=\frac{10^{-6}}{1000}=10^{-9}M$
$\therefore [{\text{H}}^{+}]$ from ${\text{H}}_2\text{O}$ cannot be neglected.
Total $\left[H^{+}\right]=10^{-9}+10^{-7}$ (neglect common ion effect on ${\text{H}}_2\text{O}$ )
$={\left(10\right)}^{-7}\left({\left(10\right)}^{-2}+1\right)$
$={\left(10\right)}^{-7}(1.01)$
$pH=-log\left(1.01\times {\left(10\right)}^{-7}\right)$
$=7-0.0043$
$=6.9957$