The pH of a 1 lit solution is 2. It is diluted with water till its pH becomes 4. How many litres of water is added?

${\mathrm{PH}}_1=2{\left[{\mathrm{H}}^{+}\right]}_1={10}^{-2}$

${\mathrm{V}}_1=1\mathrm{lit} {\mathrm{PH}}_2=4 {\left[{\mathrm{H}}^{+}\right]}_2={10}^{-4}$

${\mathrm{M}}_1{\mathrm{V}}_1={\mathrm{M}}_2{\mathrm{V}}_2$

${10}^{-2}\times 1={10}^{-4}\times {\mathrm{V}}_2$

${\mathrm{V}}_2=\frac{{10}^{-2}}{{10}^{-4}}={10}^2=100$

Added water = 100 − 1 = 99 litres