Write the chemical formulae and names of the compounds formed by the combination of:
$(i)$ $Fe^{3+}$ and $SO_{4}^{2-}$
$(ii)$ $NH_{4}^{+}$ and $CO_{3}^{2-}$

(N/A) To write the chemical formula,we use the criss-cross method where the valency of one ion becomes the subscript of the other.
$(i)$ For $Fe^{3+}$ and $SO_{4}^{2-}$:
- The valency of $Fe$ is $3$ and $SO_{4}$ is $2$.
- By crossing the valencies,we get $Fe_{2}(SO_{4})_{3}$.
- The name of the compound is Iron$(III)$ sulphate.
$(ii)$ For $NH_{4}^{+}$ and $CO_{3}^{2-}$:
- The valency of $NH_{4}$ is $1$ and $CO_{3}$ is $2$.
- By crossing the valencies,we get $(NH_{4})_{2}CO_{3}$.
- The name of the compound is Ammonium carbonate.