Q6 / 150 · MHT CET 2011 · Chemistry

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

  • A
    $6.01$
  • B
    9
  • C
    $3.5$
  • Correct answer
    D
    $6.99$

Solution

$\because \mathrm{pH}=6$ $\therefore$ $\left[\mathrm{H}^{+}\right]=10^{-6} \mathrm{M}$ After dilution $\left[\mathrm{H}^{+}\right]=\frac{10^{-6}}{1000}=10^{-9} \mathrm{M}$ $\therefore\left[\mathrm{H}^{+}\right]$ from $\mathrm{H}_{2} \mathrm{O}$ cannot be neglected. Total $\left[\mathrm{H}^{+}\right]=10^{-9}+10^{-7}$ $\begin{aligned} &=10^{-7}\left(10^{-2}+1\right) \\ &=10^{-7}(1.01) \\ \mathrm{pH} &=-\log \left(1.01 \times 10^{-7}\right) \\ &=7-0.0043 \\ &=6.9957 \end{aligned}$

Hence, the correct answer is option: 'D'.

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