1. Molecular and Empirical Formulas Percentage Composition: Mass of each element compared to the mass of the compound ( m / m ) or volume of each compared to the total volume ( v / v ) Method ( m / m ) Find the mass (molar or given) of the sample Divide the mass of each atom by the mass of the sample weight Multiply by 100% The total of all percentages must equal 100%

2. Molecular Formula Formula that gives the exact number of elements in a molecule or compound. Find the % Composition for each element in the compound. – If given % Composition: start with the % composition listed as decimals Find the contributing mass of each element – Multiply each % Composition by the sample weight of the compound to find the contributing mass Do a mass to mole conversion for each element Assemble the formula – The moles of each become the new subscripts of your compound

3. Empirical Formula Same steps as finding the molecular formula Find the sample weight of the compound Use % composition to find the contributing mass Do the mass to mole conversion for each element Divide all by the smallest and round to whole numbers Assemble the formula – The ratios expressed as whole numbers become the subscript numbers of the atoms