Empirical & Molecular Formulas

Empirical Formula: a formula which has been reduced to the lowest terms. Molecular formula: a formula of a compound in which the subscripts give the actual number of each element in the formula

Here are the four formulas being used as examples: Molecular Formula Empirical Formula H2O C2H4O2 CH2O C6H12O6 Notice two things: 1. The molecular formula and the empirical formula can be identical. 2. You scale up from the empirical formula to the molecular formula by a whole number factor.

Why chemists use it? After a compound has been analyzed for percent composition, the formula can be calculated- the empirical formula. If the actual weight of a single molecule is known then the molecular formula can be calculated as well. % comp.  Empirical Formula Actual weight  Molecular formula

How to Find Molecular Formulas 1.Calculate the mass of the empirical formula (which you have already found or it will be given to you ) 2.Divide the known molar mass by the mass of the empirical formula. 3.Multiply that number by the subscripts of the empirical formula to get the subscripts for the molecular formula.

I DO: The formula mass of a compound is 181 I DO: The formula mass of a compound is 181.5 g/mol and the empirical formula is C2HCl. What is the molecular formula?

WE DO: The empirical formula of a compound is C4H9 WE DO: The empirical formula of a compound is C4H9. If the formula mass is 114 g/mol, what is the molecular formula of this compound?

YOU DO: Find the molecular formula of Nutrasweet.  (The molar mass of NutraSweet is 294.30 g/mol) (HINT: You have already found the empirical formula of Nutrasweet)

Reminder To determine the molecular formula, you must know the compound’s empirical formula AND the molar mass of the molecular compound.