12g of alkaline earth metal gives 14.8 g of its nitride. Atomic weight of metal is ____ ?

general formula of alkaline earth metal nitride is $M_3{N}_2$

12g of metal form 14.8 g of metal nitride 

Amount of nitrogen in metal nitride is $(14.8-12) g=2.8 g$

12g of metal react with 2.8g of nitrogen 

Then 28g gram of nitrogen react with $\frac{28\times 12}{2.8}g\text{ of metal }=120 \mathrm{g}$

For 28g of nitrogen, three moles of metal atoms combine 

Therefore, atomic weight of alkaline earth element is $\frac{120}{3}=40g$