12g of an alkaline earth metal gave 14.8g of its nitride.Atomic wt of that metal is

let the metal be M with atmic wt. be x

so 3M + N₂ ---->M₃N₂

      3x                   3x+28

 so,  12/3x   = 14.8/(3x+28)

which in solving for x gives 40

the metal is calcium and its nitride is Ca₃N₂

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Mass of metal is 12g and Mass of metal nitride is 14.8gm so Mass of nitride ion is 2.8 gm.By using equivalence law: Mass of metal / Mass of Nitride ion =Equivalent Mass of metal/14 12/2.8=3x/14x=20 as x is equivalent weight by multiplying it with 2 which is valency of metal ion. We get 40. Before x is 3 is used which is valency of Nitride and 14 is Mass of N .