The pH of HCl is 1. The amount of NaOH to be added to 100 ml of such a HCl solution to get pH of 7 is

pH of HCl = 1

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

$\mathrm{HCl}+\mathrm{NaOH}\rightleftharpoons \mathrm{NaCl}+{\mathrm{H}}_2\mathrm{O}$

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

GMW of NaOH = 40

$\mathrm{M}=\frac{\mathrm{g}}{\mathrm{GMW}}\times \frac{1}{\mathrm{V}\left(\mathrm{lit}\right)}=\frac{\mathrm{g}}{40}\times \frac{1000}{100\left(\mathrm{mL}\right)}=\frac{\mathrm{g}}{4}={10}^{-1}$

$\therefore \mathrm{g}=0.4 \mathrm{g}$