c
\(\frac{1}{2} H _{2} \rightarrow H ^{+}+ e ^{\ominus}\)

\(e ^{\ominus}+ AgCl _{(s)} \rightarrow Ag _{(s)}+ Cl ^{\ominus}\)

_______________________________________________________

\(\frac{1}{2} H _{2}+ AgCl _{(s)} \rightarrow H _{( aq )}^{+}+ Ag _{(s)}+ Cl _{( aq )}^{\ominus}\)

\(E =\varepsilon^{0}-\frac{.06}{1} \log \frac{\left[ H ^{+}\right]\left[ Cl ^{\ominus}\right]}{ P _{ H _{2}}^{\frac{1}{2}}}\)

\(E =0.22-.06 \log \frac{\left(10^{-1}\right)\left(10^{-1}\right)}{1^{\frac{1}{2}}}\)

\(E =0.22+.12=34 volt\)

\(\Rightarrow\) total energy of photon will be (for Na)

\(=2.3+0.34=2.64\,eV\)

\(\Rightarrow\) stopping potential required for \(K\)

\(=2.64-2.25=0.39\) volt

\(E=\varepsilon^{0}-\frac{.06}{1} \log \frac{\left[ H ^{+}\right]\left[ Cl ^{-}\right]}{ P _{ H _{2}}^{\frac{1}{2}}}\)

as \(\left[ H ^{+}\right]=\left[ Cl ^{\ominus}\right]\) so

\(0.39=0.22-.06 \log \frac{\left[ H ^{+}\right]^{2}}{1^{\frac{1}{2}}}\)

\(0.17=+.12 pH\)

\(pH =1.4166 \Rightarrow 1.42\)