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

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

NTA AbhyasNTA Abhyas 2022

Solution:

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