100 mL solution of pH = 6 is diluted to 1000 mL. Resulting solution has pH

pH = 6

$\therefore \left[{\mathrm{H}}^{+}\right]={10}^{-6}\mathrm{M}$

$\text{ Also solution is acidic }(\mathrm{pH}<7)\text{ and dilution will not change its acidic nature.}$

$$\begin{gathered} {\left[{\mathrm{H}}^{+}\right]}_{\text{diluted }}=\frac{100\times {10}^{-6}}{1000} \\ =1\times {10}^{-7}\mathrm{M} \end{gathered}$$

[H⁺ ] from H₂O is also required while calculating resultant concentration.

$$\begin{gathered} \text{ Total }\left[{\mathrm{H}}^{+}\right]={\left[{\mathrm{H}}^{+}\right]}_{\text{diluted }}+{\left[{\mathrm{H}}^{+}\right]}_{{\mathrm{H}}_2\mathrm{O}} \\ =1\times {10}^{-7}+1\times {10}^{-7} \\ =2\times {10}^{-7}\mathrm{M} \\ \mathrm{pH}=-\log 2\times {10}^{-7} \\ =6.7 \end{gathered}$$