The $\mathrm{pH}$ of a solution is 6. Its $\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]$ is decreased by 1000 times. Its $\mathrm{pH}$ will be

It is given to find $\mathrm{pH}$ of the solution.

The $\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]$ is decreased to 1000 times now $\left[{\mathrm{H}}^{+}\right]={10}^{-9}$

This does not mean the $\mathrm{pH}$ is now 9 because ${\mathrm{H}}_3{\mathrm{O}}^{+}$ from ${\mathrm{H}}_2\mathrm{O}$ is at a higher

concentration. Take conc. of ${\mathrm{H}}_3{\mathrm{O}}^{+}$ from solvent $\left({\mathrm{H}}_2\mathrm{O}\right)$ is ${10}^{-8}$ molar.

The overall concentration of ${\mathrm{H}}^{+}={10}^{-9}+{10}^{-8}\mathrm{molar}=1.1\times {10}^{-8}={10}^{-8}$

$\left[{\mathrm{H}}^{+1}\right]={10}^{-8}$

$\mathrm{pH}=-\log \left[{\mathrm{H}}^{+}\right]=8$

Hence the correct answer (D) 8