4.6 Empirical and Molecular Formulas

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.

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.

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)

EmpiricalCH2O CH2O

Molecular CH2O CH3COOH

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

Calculating Empirical Formulae - Example

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

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.

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.