Explanation:
When \(\mathrm{\mathrm{Cr}_{2} \mathrm{O}_{7}^{-2}}\) is converted into \(\mathrm{\mathrm{CrO}_{4}^{-2}}\) the change in oxidation number of \(\mathrm{\mathrm{Cr}}\) is 3
\({\rm{In}}{\mkern 1mu} {\rm{C}}{{\rm{r}}_{\rm{2}}}{\rm{O}}_{\rm{7}}^{{\rm{2 - }}}{\rm{:oxidation}}\,{\rm{state}}\,{\mkern 1mu} {\rm{of}}{\mkern 1mu} \,{\rm{Cr}}{\mkern 1mu} \,{\rm{is}}{\mkern 1mu} \,{\rm{ + 6}}\)
\({\rm{In}}\,{\rm{CrO}}_{\rm{4}}^{{\rm{2 - }}}{\rm{:}}\,{\rm{oxidation}}\,{\rm{state}}\,{\rm{of}}\,{\rm{Cr}}\,{\rm{is}}\,{\rm{ + 3}}\)
Thus when \({{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7}\) is converted to \({{\rm{K}}_{\rm{2}}}{\rm{Cr}}{{\rm{O}}_4}\), oxidation number changes by 3.