The $\mathrm{pH}$ of a solution is 2. To one $\mathrm{ml}$ of this solution $999\mathrm{ml}$ of pure water is added. Then the $\mathrm{pH}$ of resulting solution is

 The molarity of ${\mathrm{H}}^{+}$ ions in the 1^st solution is ${10}^{-2}$ so the moles present in one $\mathrm{ml}$  is molarity ${10}^{-2}x{10}^{-3}={10}^{-5}$. in the new solution, this is the amount of ${\mathrm{H}}^{+}$ ions is 10^-5 and the molarity is 10^-5 hence the $\mathrm{pH}$ is 5.