(a) $\mathit{C}\mathit{r}=25,\left[\mathit{A}\mathit{r}\right]3{\mathit{d}}^{4}4{\mathit{s}}^2\to$ Ground state structure
${\mathit{C}\mathit{r}}^{3+}=\left[\mathit{A}\mathit{r}\right]3{\mathit{d}}^4$
There are four unpaired electrons present in ${\mathit{C}\mathit{r}}^{3+}$.
(b) ${\mathit{C}\mathit{r}}_2{\mathit{O}}_7^{2-}+3{\mathit{H}}_2\mathit{S}+8{\mathit{H}}^{+}$ $\to \underset{\mathrm{cromiumion}}{2{\mathit{C}\mathit{r}}^{3+}}+\underset{\mathrm{water}}{7{\mathit{H}}_2\mathit{O}}+\underset{\mathrm{sulphur}}{3\mathit{S}}$
(c) (i) Electronic configuration-
$\mathit{M}\mathit{n}=25,\left[\mathit{A}\mathit{r}\right]3{\mathit{d}}^{5}4{\mathit{s}}^2\to {\mathit{M}\mathit{n}}^{2+}$ $2{\mathit{e}}^{-}$
$\mathit{F}\mathit{e}=26,\left[\mathit{A}\mathit{r}\right]3{\mathit{d}}^{6}4{\mathit{s}}^2\to {\mathit{F}\mathit{e}}^{3+}$ $3{\mathit{e}}^{-}$
Mn = after loosing 2 electron $3\mathit{d}$ orbital is half filled and ${\mathit{M}\mathit{n}}^{2+}$ is stable.
$\mathit{F}\mathit{e}=\mathrm{After}$ loosing three electrons $3\mathit{d}$ orbitals is half filled and more stable. That's why, ${\mathit{F}\mathit{e}}^{3+}$ is stable at +3 oxidation state and ${\mathit{M}\mathit{n}}^{2+}$ is stable in +2 oxidation state.
(ii) $\mathit{E}°$ value of copper is +ve i.e, $0.34\mathit{V}$. This is due to presence of high enthalpy of atomization and low enthalpy of hydrogen which make it exceptionally positive.
(iii) Electronic configuration of ${\mathit{E}\mathit{u}}^{2+}\left[\mathit{X}\mathit{e}\right]4{\mathit{f}}^{7}6{\mathit{s}}^2$
It has the tendency to loose two electron and attain a stable half filled configuration. So, it oxidized by loosing 2 electrons and reduce to other species.