b
\(\quad\quad\quad{HCl}(\) aq. \()+{NaOH}\) (aq.) \(\rightarrow {NaCl}(\) aq. \()+{H}_{2} {O}(\ell)\)

\(\quad\quad\quad50\, {ml}, 1\, {M} \quad 30\, {ml}, 1\, {M} \quad\quad\quad\quad-\quad\quad\quad\quad\quad-\)

\({t}=0 \,\quad 50 \,{~mm} \,\quad\quad 30\, {~mm}\)

\({t}=\infty\quad 20\, {~mm} \quad\quad\quad\, -\)

\([{HCl}]=\frac{20}{80}=\frac{1}{4}\, {M}=2.5 \times 10^{-1}\, {M}\)

\({pH}=-\log 2.15 \times 10^{-1}=1-0.3979=0.6021\)

\({pH}=6021 \times 10^{-4}\)