How many liter of water must be added to 1 L of an aqueous solution of HCl with a pH of 1 to create an aqueous solution with pH of 2?

$\begin{array}{r}\mathrm{pH}=1\Rightarrow \left[{\mathrm{H}}^{+}\right]={10}^{-1}\mathrm{M}\\ \mathrm{pH}=2\Rightarrow \left[{\mathrm{H}}^{+}\right]={10}^{-2}\mathrm{M}\end{array}$

To change the solution having ${10}^{-1}{\mathrm{MH}}^{+}$ ions to ${10}^{-2}{\mathrm{MH}}^{+}$ the solution will have to be diluted 10 times. Thus 9 L of water will have to be added to 1 L of the original solution.