Empirical & Molecular Formula SCH4C Ms. Richardson

Empirical & Molecular Formula In Dalton’s atomic theory, he stated that atoms will combine with one another in simple, whole number ratios to form compounds. An example of a molecular formula (the actual formula) is C6H6. This is Benzene. An example of a simple whole number ratio, or the empirical formula for Benzene would be CH.

Comparing molecular and empirical formulas Name of Compound Molecular Formula Empirical Formula hydrogen peroxide H2O2 HO glucose C6H12O6 CH2O benzene C6H6 CH ethyne C2H2 water H2O

Determining Empirical Formula You already know how to calculate percent composition. In this section, you will use percent composition to find the formula of the compound. These are the 3 steps to determining empirical formula. Determine the mass of the elements present. Determine the moles of the elements present. Divide the # of moles of each element by the lowest number of moles

Empirical formula example What is the empirical formula of a compound with 79.9 g of Cu and 20.1 g of S? “Percent to mass Mass to mole Divide by small Multiply ‘til whole”

Another example Calculate the empirical formula of a compound that has 85.6% carbon and 14.4% hydrogen. HINT: Assume that the percentage composition is based on 100g of the compound.

A tricky Example What happens when the mole ratio does not give you nice whole numbers? The percent composition of a fuel is 81.7% carbon and 18.3% hydrogen. What is the empirical formula?

Getting to whole numbers 0.2, 1.2, 2.2, 3.2……. Multiply by 5 0.25, 1.25, 2.25 … …Multiply by 4 0.3, 1.3, 2.3, 3.3…… Multiply by 3 0.5, 1.5, 2.5, 3.5…… Multiply by 2

Molecular Formula Once we have determined the empirical formula (the smallest whole number ratio), we can determine the compound’s actual formula: Determine the molar mass of the empirical formula Divide the given molar mass by the molar mass of the empirical formula.