Which of the following pair of compounds demonstrates the law of multiple proportions?

What is number of atoms present in $$2.24 \mathrm{~dm}^3 \mathrm{~NH}_{3(\mathrm{~g})}$$ at STP?

Calculate the percent atom economy when a product of formula weight $$175 \mathrm{u}$$ is obtained in a chemical reaction using $$225 \mathrm{u}$$ formula weight reactant.

According to reaction,

$$\mathrm{Mg}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{MgCl}_2(a q)+\mathrm{H}_2(g) \uparrow .$$

Calculate the mass of $$\mathrm{Mg}$$ required to liberate $$4.48 \mathrm{~dm}^3 \mathrm{~H}_2$$ at STP?

(Molar mass of $$\mathrm{Mg}=24 \mathrm{~g} \mathrm{~mol}^{-1}$$)