EMPIRICAL AND MOLECULAR

FORMULASUnit 7: Moles

WHAT ARE MOLECULAR FORMULAS? Molecular formulas tell the actual number of atoms of each element in a compound Example: Glucose – C6H12O6 (one molecule has 6 carbons 12 hydrogens and 6 oxygens)

WHAT ARE EMPIRICAL FORMULAS? Empirical formulas are the simplest, whole number ratio of elements in a compound. It’s the reduced chemical formula Example: Benzene – C6H6 (molecular formula) CH (empirical formula) Water – H2O (molecular formula) H2O (empirical formula) The empirical formula CAN be the same as a molecular formula

EMPIRICAL OR MOLECULAR FORMULA? Dinitrogen tetroxide

Molecular:________________________

Empirical: ______________________

Tetraphosphorus decoxide

Molecular: _______________________

Empirical: _______________________

HOW DO YOU CALCULATE EMPIRICAL FORMULAS? Remember this rhyme…

Percent to Mass Mass to Mole

Divide by Small Multiply ‘til Whole

HOW DO YOU CALCULATE EMPIRICAL FORMULAS? 1. Assume you have 100g of the sample. Multiply the percent of each element by 100g to determine the amount in grams of each element. 2. Convert grams to moles for each element 3. Divide all answers by the smallest number of moles. 4. If needed, multiply all numbers by the same number to get WHOLE numbers. Your final answer cannot have decimals. 5. The whole numbers are the subscripts of the empirical formula

PRACTICE A compound is found to be 34.53% zinc, 14.79% nitrogen, and 50.68% oxygen. Calculate the empirical formula for this compound.

HOW DO YOU FIND THE MOLECULAR FORMULA FROM THE EMPIRICAL FORMULA? 1. Calculate molar mass for the empirical formula 2. Find the molar mass given in the problem. 3. Find the “magic number” n

4. Multiply each subscript in the empirical formula by n (“magic number”) 5. Write the formula with the metal first OR in the order in which it was given if there is not a metal.

PRACTICE The empirical formula for a compound is CHO. The molar mass is 116.1 g/mol. Determine the molecular formula.