1. 3.4 Empirical and Molecular Formulas

2. Empirical and Molecular Formulas 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.

3. Empirical and Molecular Formulas 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.

4. Empirical and Molecular Formulas 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

5. Calculating Empirical Formulae 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

6. Calculating Empirical Formulae - Example A compound is 50.91% Zn, 16.04% P, 33.15% O. Find its Empirical Formula.

7. Calculating Molecular Formulae 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.

8. Determining Molecular Formula - Example 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.