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

(N/A) $(i)$ Magnesium has a valency of $+2$ and chloride has $-1$. By criss-cross method,the formula is $MgCl_2$.
$(ii)$ Calcium has a valency of $+2$ and oxide has $-2$. The ratio is $1:1$,so the formula is $CaO$.
$(iii)$ Copper $(II)$ has a valency of $+2$ and nitrate has $-1$. By criss-cross method,the formula is $Cu(NO_3)_2$.
$(iv)$ Aluminium has a valency of $+3$ and chloride has $-1$. By criss-cross method,the formula is $AlCl_3$.
$(v)$ Calcium has a valency of $+2$ and carbonate has $-2$. The ratio is $1:1$,so the formula is $CaCO_3$.