7.
常温下,在溶液中可发生以下反应:\(\rm{① H^{+} + Z^{-} + XO_{4}^{2-} → X^{2+} + Z_{2} + H_{2}O(}\)未配平\(\rm{)}\),\(\rm{② 2M}\)\(\rm{{\,\!}^{2+}}\) \(\rm{+ R}\)\(\rm{{\,\!}_{2}}\) \(\rm{= 2M}\)\(\rm{{\,\!}^{3+}}\) \(\rm{+ 2R}\)\(\rm{{\,\!}^{-}}\),\(\rm{③ 2R}\)\(\rm{{\,\!}^{-}}\) \(\rm{+ Z}\)\(\rm{{\,\!}_{2}}\) \(\rm{= R}\)\(\rm{{\,\!}_{2}}\) \(\rm{+ 2Z}\)\(\rm{{\,\!}^{-}}\)。由此判断下列说法正确的是