Question

which of the following ionic compounds is incorrectly named?
ca3(po4)2, calcium phosphate
cu2s, copper(i) sulfide
fe2o3, iron(iii) oxide
(nh4)2so3, ammonium sulfite
pbo2, lead(ii) oxide

Explanation:

Step1: Recall naming rules for ionic compounds

Ionic compounds are named based on the cation (positive - ion) and anion (negative - ion). For metals with variable oxidation states, the oxidation state is indicated in Roman numerals in parentheses. Polyatomic ions have specific names.

Step2: Analyze each option

• $\ce{Ca3(PO4)2}$: Calcium ($\ce{Ca^{2+}}$) and phosphate ($\ce{PO4^{3 - }}$) combine correctly to form calcium phosphate.
• $\ce{Cu2S}$: Copper has a + 1 oxidation state here, so it is named copper(I) sulfide correctly.
• $\ce{Fe2O3}$: Iron has a + 3 oxidation state in this compound, so it is named iron(III) oxide correctly.
• $\ce{(NH4)2SO3}$: The anion is sulfite ($\ce{SO3^{2 - }}$), not sulfate ($\ce{SO4^{2 - }}$). The correct name should be ammonium sulfite, but the naming convention is mis - applied here as it might be confused with ammonium sulfate.
• $\ce{PbO2}$: Lead has a + 4 oxidation state here, so it should be lead(IV) oxide, not lead(II) oxide.

Answer:

$\ce{PbO2}$, lead(II) oxide is incorrectly named. The correct name should be lead(IV) oxide.