RecallIf we know MF, we can easily determine EF.

e.g. pentane C5H10 (MF)

CH2 (EF)

But, given EF can you determine the MF?

e.g. CH2O (EF) ? (MF) See p269 table 6.1 for possibilities.Name MF Whole-

number multiple

M(g/mol)

Use

Formaldehyde

CH2O 1 30.03 Disinfectant

Acetic acid

C2H4O2 2 60.05 Vinegar

Lactic acid

C3H6O3 3 90.08 In sour milk

Erythrose C4H8O4 4 120.10 In sugar

Ribose C5H10O5 5 150.13 Nucleic acids

Glucose C6H12O6 6 180.16 Energy in cells

Additional information is needed:molar mass (M) of the compound is needed

e.g. CH2O (EF) ? (MF)

from EF M = 30 g/mol from expt with compound M = 150 g/mol

ratio: 150 g/mol = 530 g/mol

Therefore: x CH2O C5H10O5 (EF) (MF) ribose

5

Using Molar Mass1. The empirical formula (EF) of hexane C3H7. In an experiment with a mass spectrometer, a chemist determines the molar mass of hexane is 86.2 g/mol. What is the molecular formula (MF)?

2. The EF of an unknown sample is CH2O. In an experiment it is found that the molar mass of the sample is 121 g/mol. What is the molecular formula (MF)?

Text p279 # 1, 4, 6, 11, 12, 16 **Quest on Chapter 6 in 4 days!