Question

$12g$ of an alkaline earth metal gives $14.8g$ of its nitride. The atomic mass of metal is:

• A. 12
• B. 24
• C. 20
• D. 40

Answer 1

The correct option is D 40

The balanced chemical reaction for the formation of nitride of alkaline earth metal M is as given below:
$3M+N_2\to M_3{N}_2$

Let m be the molar mass of metal.
Molecular weight of the nitride $M_3{N}_2$ = 3m + 28

The number of moles of nitride obtained is $\frac{14.8}{3m+28}$

The number of moles of metal used is $\frac{12}{m}$

As per the stoichiometry, 3 moles of metal will give one mole of nitride.

Hence, $\frac{12}{m}$ moles of metal will give $\frac{12}{3m}=\frac{4}{m}$ of the nitride.

But this is equal to $\frac{14.8}{3m+28}$

Hence, $\frac{14.8}{3m+28}=\frac{4}{m}$
Hence, $m=40$.

Thus, the molar mass of metal is 40 g/mol.