How many atoms are present in a $(i)$ $H_2S$ molecule and $(ii)$ $PO_4^{3-}$ ion?

Calculate the number of aluminium ions present in $0.051 \, g$ of aluminium oxide.
[Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of $Al = 27 \, u$.]

Which has more number of atoms,$100\,g$ of sodium or $100\,g$ of iron (given,atomic mass of $Na = 23\,u, Fe = 56\,u$)?

Give the names of the elements present in the following compounds:
$(a)$ Quick lime
$(b)$ Hydrogen bromide
$(c)$ Baking powder
$(d)$ Potassium sulphate

What are polyatomic ions? Give examples.

Calculate the number of molecules of sulphur $(S_8)$ present in $16 \, g$ of solid sulphur.