Question

Which of the following is amphoteric oxide?$\text{M}{{\text{n}}_{\text{2}}}{{\text{O}}_{7}}$,$\text{Cr}{{\text{O}}_{\text{3}}}$, $\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$,$\text{CrO}$, ${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{5}}}$, ${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{4}}}$(i)${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{5}}}$, $\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$(ii)$\text{M}{{\text{n}}_{\text{2}}}{{\text{O}}_{7}}$, $\text{Cr}{{\text{O}}_{\text{3}}}$(iii)$\text{CrO}$, ${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{5}}}$(iv)${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{5}}}$, ${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{4}}}$

In $M{{n}_{2}}{{O}_{7}}$, $\text{M}{{\text{n}}^{\text{7+}}}$ ion present in their maximum oxidation state. It can only gain electrons, so this metal oxide will be acidic in nature.
Chromium metal ions show variation in oxidation state from +2 to +6. In $\text{Cr}{{\text{O}}_{\text{3}}}$, $\text{C}{{\text{r}}^{\text{+6}}}$ ion present in maximum oxidation state, so this oxide will be acidic in nature. However in $\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$, $\text{C}{{\text{r}}^{\text{3+}}}$ present in middle oxidation state. $\text{C}{{\text{r}}^{\text{3+}}}$ Ion is amphoteric in nature. $\text{C}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{3}}}$ is insoluble in water. It soluble in acid form ${{\text{ }\!\![\!\!\text{ Cr(}{{\text{H}}_{\text{2}}}\text{O}{{\text{)}}_{\text{6}}}\text{ }\!\!]\!\!\text{ }}^{\text{3-}}}$ salt and also reacts with base to form salt of ${{\text{ }\!\![\!\!\text{ Cr(OH}{{\text{)}}_{\text{6}}}\text{ }\!\!]\!\!\text{ }}^{\text{3-}}}$. In $\text{CrO}$, $\text{C}{{\text{r}}^{\text{2+}}}$ ion present in the lowest oxidation state. So, this is a basic metal oxide.
In${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{5}}}$, ${{\text{V}}^{\text{+5}}}$ ion is present in its highest oxidation state. However ${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{5}}}$ is amphoteric oxide but mainly acidic. ${{\text{V}}_{\text{2}}}{{\text{O}}_{\text{5}}}$ Reacts with alkalis as well as acids gives $\text{VO}_{\text{4}}^{\text{3-}}$ and$\text{VO}_{\text{2}}^{\text{+}}$.