Empirical and Molecular Formulas

2011

Empirical and Molecular Formulas

• An empirical formula shows the simplest whole number ratio of atoms of each element in a compound

• A molecular formula shows the actual number of atoms of each element in a compound

Example: Glucose

• Glucose has a molecular formula of:– C6H12O6

• It has an empirical formula of:– CH2O

Formulas cont’d

• A molecular formula is always a multiple of empirical formula

• A compound has ONE molecular formula

• Many compounds have the same empirical formula

Example• An compound contains 28.4g of copper and

71.6g of bromine. Calculate the empirical formula of the compound:

1. Convert all grams to moles2. Divide all numbers of moles by the smallest #

present (do a ratio!)3. Multiply all numbers by the smallest # that will

make them WHOLE NUMBERS

Example What is the empirical formula for a compound

that contains: 77.3% Silver, 7.4% phosphorous and 15.3% O?

Convert all grams to moles Divide all numbers of moles by the smallest #

present (do a ratio!) Multiply all numbers by the smallest # that will make

them WHOLE NUMBERS

Example What is the empirical formula for a compound

that contains 5.51 g of cobalt and 4.49 g of sulfur?

Convert all grams to moles Divide all numbers of moles by the smallest #

present (do a ratio!) Multiply all numbers by the smallest # that will make

them WHOLE NUMBERS

Molecular formulas from empirical formulas

• A white powder is analyzed and found to have an empirical formula of P2O5. It has a molar mass of 283.88 g/mol. What is the molecular formula for this compound?