 How do we find a chemical formula for and unknown substance? › Identify elements & # of atoms of each element › Determine % composition by mass › Convert mass values  moles › Formula = empirical formula  Empirical Formula › Simplest whole-number ratio of atoms or ions in a compound.

 An Empirical Formula does not necessarily provide correct information about the number of atoms in a molecule – only the ratio of atoms.  Different molecules may have the same % composition but contain different numbers of atoms  A Molecular Formula is needed to tell us the number and kind of atoms in a molecule.

 A Molecular Formula represents the number of atoms of each element that make up a single molecule of that compound.  Example Formaldehyde (poison) Acetic Acid (vinegar) Empirical CH 2 O MolecularCH 2 OCH 3 COOH

 Convert % comp. data to mass data, assuming total mass is 100g.  Determine number of moles of each element, by dividing mass by molar mass  Convert # of moles into whole numbers by dividing each amount in moles by the smallest number  Use these as subscripts in formula

 A compound is 50.91% Zn, 16.04% P, 33.15% O. Find its Empirical Formula.

 Determine Molar Mass of Empirical Formula  Determine Molar Mass of actual compound › Done experimentally – mass spec.  Divide experimental molar mass by empirical molar mass & multiply answer by each subscript.

 A compound with the empirical formula CH was analyzed using a mass spectrometer. Its molar mass was found to be 78 g/mol. Determine the molecular formula.