From the following oxides, how many are amphoteric: ZnO, Al2O3, PbO, SnO2, BeO, Cr2O3, CO, N2O, B2O3, CaO, MgO, Mn2O7?

Amphoteric oxides dissolve in both acids and strong alkalis. $\mathrm{ZnO}$ gives $\mathrm{Zn^{2+}}$ with acid and $\mathrm{[Zn(OH)_4]^{2-}}$ with alkali; $\mathrm{Al_2O_3}$ gives $\mathrm{Al^{3+}}$ and $\mathrm{[Al(OH)_4]^-}$; $\mathrm{PbO}$ gives $\mathrm{Pb^{2+}}$ and plumbite; $\mathrm{SnO_2}$ dissolves in conc. acids and fused alkali to give stannate; $\mathrm{BeO}$ gives $\mathrm{Be^{2+}}$ and beryllate; $\mathrm{Cr_2O_3}$ gives $\mathrm{Cr^{3+}}$ and chromite. That makes six. The others are not amphoteric: $\mathrm{CO}$ and $\mathrm{N_2O}$ are neutral; $\mathrm{B_2O_3}$ and $\mathrm{Mn_2O_7}$ are acidic (boric acid and permanganic acid); $\mathrm{CaO}$ and $\mathrm{MgO}$ are basic. Hence the number of amphoteric oxides is 6.