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 1 of 3: Gas identification

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Intuition

Active reducing metals like Zn or Mg can reduce dilute HNO3 all the way to oxidation state -3 (ammonium).

\ce{NH4+ + OH- -> NH3 ^ + H2O}; \quad n(\ce{NH3}) = \dfrac{1.12}{22.4} = 0.050\ \text{mol}

Analysis

The gas evolved upon heating with alkali is ammonia ( $\ce{NH3}$ ), proving that the metal reduced dilute nitric acid to ammonium nitrate ( $\ce{NH4NO3}$ ).