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 $Al = 27 \, u$.]

Write down the names of compounds represented by the following formulae:
$(i) Al_{2}(SO_{4})_{3}$
$(ii) CaCl_{2}$
$(iii) K_{2}SO_{4}$
$(iv) KNO_{3}$
$(v) CaCO_{3}$

Write down the chemical formulae of:
$(i)$ Sodium oxide
$(ii)$ Aluminium chloride
$(iii)$ Sodium sulphide
$(iv)$ Magnesium hydroxide

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

Write the chemical formulae of the following:
$(i)$ Magnesium chloride
$(ii)$ Calcium oxide
$(iii)$ Copper nitrate
$(iv)$ Aluminium chloride
$(v)$ Calcium carbonate.