Multiple Choice
When naming ionic compounds, in which situation must you use a Roman numeral in the compound's name?
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3. Chemical Reactions
Naming Ionic Compounds
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Which of the following is the correct chemical formula for iron(II) phosphite?
A
FePO3
B
Fe2(PO3)3
C
Fe3(PO3)2
D
Fe(PO3)2
Verified step by step guidance1
Identify the charge on the iron(II) ion. The Roman numeral II indicates that iron has a +2 charge, so the ion is Fe^{2+}.
Determine the charge on the phosphite ion. The phosphite ion is PO_3^{3-}, which carries a -3 charge.
Balance the total positive and negative charges to form a neutral compound. Since Fe^{2+} has a +2 charge and PO_3^{3-} has a -3 charge, find the smallest whole number ratio of Fe^{2+} to PO_3^{3-} that results in zero net charge.
Use the least common multiple of the charges (2 and 3) to balance the charges. The least common multiple is 6, so multiply Fe^{2+} by 3 (3 × +2 = +6) and PO_3^{3-} by 2 (2 × -3 = -6).
Write the chemical formula using the ratio found: 3 iron(II) ions and 2 phosphite ions, which gives Fe_3(PO_3)_2.
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