Problem 6

A sample of 13.0 g of an unknown metal M was treated with an excess of dilute nitric acid. Excess hot alkaline solution was then added to the resulting mixture, which evolved 1.12 dm

^3

of a gas measured at STP. Identify the metal M.

Step 2 of 3: Redox stoichiometry

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8\ce{M} + 10n\ce{HNO3} -> 8\ce{M(NO3)_n} + n\ce{NH4NO3} + 3n\ce{H2O}

Analysis

Each atom of M loses $n$ electrons: $\ce{M -> M^{n+} +} n e^-$ . Nitrogen in nitrate (+5) is reduced to ammonium (-3), accepting 8 electrons: $\ce{NO3- + 10H+ + 8e- -> NH4+ + 3H2O}$ . Balancing electron transfer gives an 8 : $n$ ratio between M and $\ce{NH4NO3}$ .