The $\mathrm{pH}$ of a 1 lit solution is 2 . It is diluted with water till its $\mathrm{pH}$ becomes 4. How many litres of water are added?

$p^H=2,\left(H^{+}\right)={10}^{-2}$

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

$V_2=\frac{10^{-2}}{10^{-4}}=10^2=100$

use of water $=V_2-V_1$

$-100-1 = 99$

Hence the correct answer (A) 99