Chapter 5 · Molecules and Compounds
5.3Chemical Formulas: How to Represent Compounds
10 min · two checks
Predict
What is the difference between a molecular formula and an empirical formula?
The idea
- Read subscripts, parentheses, and empirical versus molecular formulas.
- List the atoms in a formula that contains parentheses.
A chemical formula lists each element’s symbol and a subscript for the count. No subscript means one. H₂O has two H and one O. Parentheses multiply everything inside: Ca(NO₃)₂ has one Ca, two N, and six O. A molecular formula gives the actual count in a molecule. An empirical formula gives the simplest integer ratio. Glucose is C₆H₁₂O₆ molecular and CH₂O empirical. The empirical formula is what composition data can give you before you know the molecule’s size.
Structural formulas and models show connections, which a bare formula hides. C₂H₆O could be ethanol or dimethyl ether; both share a molecular formula and differ in structure. For this section, be fluent at counting atoms, including inside polyatomic groups. Charge, if present, belongs to the whole ion: SO₄²⁻ is one sulfate group with a 2− charge, not a sulfur with subscript 4 and a random minus sign.
Keep these
- Subscripts count atoms. Parentheses multiply the group.
- Empirical = simplest ratio. Molecular = actual molecule.
- Same molecular formula can still mean different structures.