How many moles of $BaCO_3$ will contain $1.5$ moles of oxygen atoms?

Calculate the number of $Ca^{2+}$ ions in $222 \ g$ of anhydrous calcium chloride $(CaCl_2)$? (Atomic mass: $Ca = 40 \ u$,$Cl = 35.5 \ u$)

The number of atoms in $4.25 \ g$ of $NH_3$ is approximately

In which of the following pairs do $1 \text{ g}$ of each contain the same number of molecules?
$(I)\, N_2O$ and $CO$               $(II)\,N_2$ and $CO_2$
$(III)\, N_2$ and $CO$                $(IV) \,N_2O$ and $CO_2$

What is the total number of moles of $C$ atoms and $H$ atoms respectively present in $n$ moles of the following molecule?

Calculate the number of atoms in each of the following:
$(i)$ $52$ moles of $Ar$
$(ii)$ $52$ $u$ of $He$
$(iii)$ $52$ $g$ of $He$.