a)Chất B là \(FeC{l_3}\) hoặc \(F{e_2}{(S{O_4})_3}\) hoặc \(Fe{(N{O_3})_3}\).
\(Fe + 2F{e^{3 + }} \to 3F{e^{2 + }}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Cu + 2F{e^{3 + }} \to 2F{e^{2 + }} + C{u^{2 + }}\)
b) Chất B là \(AgN{O_3}\)
\(\eqalign{
& Fe + 2AgN{O_3}\buildrel {} \over
\longrightarrow Fe{(N{O_3})_2} + 2Ag \downarrow \cr
& Cu + 2AgN{O_3}\buildrel {} \over
\longrightarrow Cu{(N{O_3})_2} + 2Ag \downarrow \cr} \)