Question

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

Answer 1

M3N2 is the general molecular formula of nitrides of alkaline earth metals.
12 g of the metal gives 14.8 g of its nitride.
so mass of nitrogen combining with 12g of the metal=14.8-12
                                                                                          =2.8g
                                                                                           =2.8/14 moles
                                                                                          =0.2 mole
from this we get the mass of the metal combining with 2 moles of nitrogen=12*10g
              =120g
120g is the weight of 3 moles of the metal
therefore the mass of 1 mole of the metal=120/3=40g
mass of 1 atom of the metal=40g/(6.022*10^23)
                                                =6.64*10^-23g
                                                =(6.64*10^-23)/(1.67*10^-24)
                                                =40u
 atomic weight of the metal is 40u