In a compound of magnesium (Mg = 24) and nitrogen (N = 14), 18 g of magnesium combines with 7g of nitrogen. Deduce the simplest formula by answering the following questions.

(a) How many gram-atoms of magnesium are equal to 18g?

(b) How many gram-atoms of nitrogen are equal to 7g of nitrogen?

(c) Calculate simple ratio of gram-atoms of magnesium to gram-atoms of nitrogen and hence the simplest formula of the compound formed.

(a) Gram atom = $\dfrac{\text{Mass of element}}{\text{Atomic mass}}$

∴ g atom of magnesium = $\dfrac{18}{24}$ = $\dfrac{3}{4}$

Hence, $\dfrac{3}{4}$ gram atoms of magnesium are equal to 18g of magnesium.

(b) g atom of nitrogen = $\dfrac{7}{14}$ = $\dfrac{1}{2}$

Hence, $\dfrac{1}{2}$ gram atoms of nitrogen are equal to 7g of nitrogen.

(c) simple ratio of gram-atoms of magnesium to gram-atoms of nitrogen

= $\dfrac{\dfrac{3}{4}}{\dfrac{1}{2}}$ = $\dfrac{3}{2}$ = magnesium : nitrogen

So, the formula is Mg₃N₂