How do you determine the chemical formula of an ionic compound from the name of the compound?

Question

(assume only representative elements for your explanation.) I need to know simply by looking at the name of the ionic compound how to determine the chemical formula.

Answer 1

Hi Taylor H.,

It does depend a bit on how precise your compound name is. If it's per IUPAC, that will give you the cation(s), and any ambiguous oxidation state(s) associated, and then the anion(s). But you may be given less information than that, perhaps. Bear in mind that "mono" prefix is omitted, for anions, when otherwise unambiguous, but other prefixes, for anions, are generally expressed. Cations are usually NOT so specified, so you may have to think a bit, work up an empirical formula, and then consider if that is reasonable.

So if your compound were mercury(I) chloride, you would use that information to assemble the formula. In this case, it does help to know that mercury(I) is designated as an Hg₂²⁺ unit, so that your total compound would be Hg₂Cl₂ . But mercury(II) chloride has the Hg²⁺ as discrete units, so the formula is HgCl₂ .

It will be necessary to use your knowledge of usual cation and anion charges to arrive at your stoichiometry! Thus, iron(III) oxide is Fe₂O₃ (in the anhydrous form), though commonly found as Fe₂O₃ • 3H₂O when wet.

And that's before worrying about shared oxyanion frameworks, as in polyphosphates or linear, sheet, and higher dimension polysilicates. Or metal complexes with both intermetal bonds and bonds to other ligands. Worry about those when you get to them!

-- Cheers, -- Mr. d.