On adding which of the following, the \(\mathrm{pH}\) of \(20 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{HCl}\) will not alter?

Milliequivalents of \(20 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{HCl}=\mathrm{N}_{1} \mathrm{~V}_{1}\)
\[
=20 \times 0.1=2
\]
(a) \(\mathrm{pH}\) of distilled water \(=7\)
\[
\begin{aligned}
\therefore\left[\mathrm{H}^{+}\right]=10^{-\mathrm{pH}}=& 10^{-7} \mathrm{M} \\
\therefore \text { milliequivalents } &=\mathrm{NV} \quad \text { [for } \mathrm{HCl}, \mathrm{N}=\mathrm{M} \text { ] } \\
&=10^{-7} \times 20=2.0 \times 10^{-6}
\end{aligned}
\]
(b) Milliequivalents of \(\mathrm{NaOH}=0.1 \times 1=0.1\)
(c) \(\mathrm{pH}\) of \(\mathrm{HCl}=1\)
\(\therefore \quad\left[\mathrm{H}^{+}\right]=10^{-1}=0.1 \mathrm{M}\)
\(\therefore\) Milliequivalents \(=0.1 \times 500=50\)
(d) Milliequivalents of \(\mathrm{HCl}=1 \times 1=1\)
\(\because\) Milliequivalents of \(500 \mathrm{~mL}\) of \(\mathrm{HCl}\) having \(\mathrm{pH}=1\) is more than that of \(20 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{HCl}\), therefore adding this, \(\mathrm{pH}\) of \(0.1 \mathrm{~N} \mathrm{HCl}\) solution, does not alter.