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