Chem-To-Go Lesson 26 Unit Six EMPIRICAL AND MOLECULAR FORMULAS You’ll need a periodic table and a calculator!

Chemical formulas Empirical (reduced subscripts) Molecular (actual formula; may or may not be reduced) EXAMPLE: C 6 H 12 O 6 = the molecular formula for glucose Is that formula an empirical formula? If not, what is the empirical formula for glucose?

 SCENARIO: An unknown chemical is found at a crime scene. A crime scene tech collects the sample and takes it back to the forensic lab. The chemist runs the sample through a mass spec, and the machine reports the % composition of the unknown sample.  Mass spectrometer analysis:  40.9 % Carbon  4.58% Hydrogen  54.5% Oxygen  Molar mass of 180 grams/mole CONNECTION TO % COMPOSITION BUT WHAT IS THE COMPOUND? Millions of compounds contain C, H, and O. We can convert the % composition into a chemical formula!

 Mass spectrometer analysis:  40.9 % Carbon  4.58% Hydrogen  54.5% Oxygen  Molar mass of 180 grams/mole FINDING A CHEMICAL FORMULA Step One: FIND EMPIRICAL FORMULA % to gram Gram to mole ÷ by the smallest X until whole Learn the rhyme! Is this the ACTUAL (MOLECULAR) formula though?

SUMMARY Step One: FIND EMPIRICAL FORMULA % to gram Gram to mole ÷ by the smallest X until whole Learn the rhyme! Step Two: CHECK ACTUAL FORMULA Find the molar mass of the empirical formula. Does it match the given molar mass? If not, then distribute a whole number through your empirical formula. o 71.65% Cl o 24.27% C o 4.07% H o The molar mass is known to be g