Question 2.12: Recognizing Incorrect Names and Formulas of Ionic Compounds ......

Solution (a) The charge of the $Ba^{2+}$ ion must be balanced by two $C_2H_3O_2^−$ ions, so the prefix di- is unnecessary. For ionic compounds, we do not indicate the number of ions with numerical prefixes. The correct name is barium acetate.
(b) Two mistakes occur here. The sodium ion is monatomic, so it does not require parentheses. The sulfide ion is $S^{2−}$, not $SO_3^{2−}$ (which is sulfite). The correct formula is $Na_2S$.
(c) The roman numeral refers to the charge of the ion, not the number of ions in the formula. $Fe^{2+}$ is the cation, so it requires one $SO_4^{2−}$ to balance its charge. The correct formula is $FeSO_4$. [$Fe_2(SO_4)_3$ is the formula for iron(III) sulfate.]
(d) Parentheses are not required when only one polyatomic ion of a particular kind is present. The correct formula is $Cs_2CO_3$.