Writing Empirical Formulas for Ionic Compounds

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Although ionic compounds are composed of both positively and negatively charged ions, the overall compound and its formula are electrically neutral. In other words:
total positive charge (from cation) = total negative charge (from anion)
Forgetting to follow this rule is one of the most common mistakes that students make when writing formulas for ionic compounds.
The empirical formula for an ionic compound indicates the smallest whole number ratio of cations and anions needed to produce an electrically neutral compound. The empirical formula is written with the cation first followed by the anion.
To write the empirical formula for an ionic compound:
  1. Identify the cation.
    • Cation is written first in the name of the compound.
  2. Write the correct formula and charge for the cation.
  3. Identify the anion.
    • Anion is written last in the name of the compound.
  4. Write the correct formula and charge for the anion.
  5. Combine the cation and anion to produce an electrically neutral compound.
    • If the charges on the cation and anion are equal in magnitude (i.e. +1/-1, +2/-2, +3/-3), combine the cation and anion in a 1:1 ratio.
    • If the charges on the cation and anion are NOT equal in magnitude, use the charge on the cation as the subscript for the anion. Use the charge on the anion (omitting the negative sign) as the subscript for the cation.
    • Place parentheses around a polyatomic ion if you need more than one of them in the final formula.
    • Do not show the charges of the ions when you write the final formula for the compound.
  6. Make sure that the subscripts for the cation and anion are the smallest whole number ratio.
 

Example: Write the empirical formula for sodium carbonate.

  1. The cation is written first: sodium ion (Na+)
  2. The anion is written last: carbonate ion (CO32-)
  3. Combine them to form an electrically neutral compound:
    • Since the charges are not equal in magnitude the charge of sodium ion (+1) becomes the subscript for the carbonate ion. The charge of the carbonate ion (2-) becomes the subscript for the sodium ion.
  4. The final formula is Na2CO3.
    • Since only one carbonate ion is needed, the subscript "1" is omitted from the formula.
    • Also notice that the charges of the ions are NOT shown when writing the final formula.
Na+
CO32-
Na+

 

Example: Write the empirical formula for aluminum sulfate.

  1. The cation is written first: aluminum ion (Al3+)
  2. The anion is written last: sulfate ion (SO42-)
  3. Combine them to form an electrically neutral compound:
    • Since the charges are not equal in magnitude, the charge on the aluminum ion (+3) becomes the subscript for the sulfate ion. The charge on the sulfate ion (-2) becomes the subscript for the aluminum ion.
  4. The correct formula is Al2(SO4)3.
    • Notice that parentheses are placed around the sulfate ion to indicate that three sulfate ions are needed.
Al3+
Al3+
Al3+
SO42-
SO42-

Example: Write the empirical formula for lead (IV) oxide.

  1. The cation is written first: lead (IV) ion (Pb4+)
  2. The anion is written second: oxide ion (O2-)
  3. Combine them to form an electrically neutral compound.
    • Since the charges are not equal in magnitude, the charge on the lead (IV) ion (+4) becomes the subscript for the oxide ion. The charge on the oxide ion (-2) becomes the subscript for the lead (IV) ion.
  4. The initial formula that is written would be Pb2O4. Notice, however, that this is not the empirical formula. Both subscripts can be divided by 2 giving the correct empirical formula, PbO2.
Pb4+
O2-
O2-

Practice Problems

Write the correct empirical formula for each of the following compounds:

  1. magnesium carbonate
  2. aluminum hydroxide
  3. iron (III) chloride
  4. sodium bicarbonate
  5. ammonium phosphate
  6. tin (IV) sulfate
  7. potassium perchlorate

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