Chem-To-Go Lesson 26Unit Six
EMPIRICAL AND MOLECULAR
FORMULASYou’ll need a periodic table
and a calculator!
Chemical
formulas
Empirical(reduced
subscripts)
Molecular (actual formula; may
or may not be reduced)
• EXAMPLE: C6H12O6 = 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 wholeLearn 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% Clo 24.27% Co 4.07% Ho The molar mass is
known to be 98.96 g