SL Chem - Unit #2: Empirical and Molecular Formula

Molecular Formula

Represents the actual number of atoms in the compound

• Not simplified - A multiple of it’s empirical formula

Empirical/Simplest Formula

The lowest whole number ratio of atoms

• Simplified - gives elemental composition as percentages/ratios

Calculation Steps for Empirical Formula

1. Assume that there is 100g of substance unless given a number

2. Calculate the moles of each substance

3. Determine the molar ratio of the compound by dividing all the moles by the smallest mole value

   • ratios are ALWAYS WHOLE NUMBERS so if it isn’t a whole number keep multiplying all the values by something until it is a whole number

4. Then the ratios that you get for each element is written as their subscripts

Calculation Steps for Molecular Formula

1. Unless already provided with the empirical formula you need to solve for that first

2. Using the ratios in the empirical formula determine the molar mass

3. Normally in these questions a mass of the compound is given, you divide that mass by the empirical formula mass

   • Ratio = (molecular formula mass)/(Empirical formula mass)

4. Multiply the empirical formula subscripts by this ratio and you get your molecular formula!!