The Molecular Formula

1
The Molecular Formula

• Chemistry 11
• Ms. McGrath

2
The Molecular Formula

• The empirical formula CH has a molar mass
• of 13 g/mol.
• Acetylene, C2H2, and benzene, C6H6 both
• have the empirical formula CH.
• Suppose its been determined through mass
• spectrometry that a sample has a molar mass
• of 78 g/mol. The compound must be C6H6,
• since 6 x 13 g 78 g.

3
Determining a Molecular Formula

• The empirical formula for ribose is
• CH2O. In an experiment, using a mass
• spectrometer, the molar mass of ribose
• was determined to be 150 g/mol. What
• is the molecular formula for ribose?

4
Determining a Molecular Formula

• The molar mass of the empirical formula
• CH2O is 30 g/mol.
• The molar mass of ribose is 150 g/mol.
• 150 g/mol 5
• 30g/mol
• Molecular formula subscripts 5 x empirical
  formula
• subscripts
• C1 x
  5H2 x 5O1 x 5
• C5H10O5

5
Determining a Molecular Formula

• You try page 97, 17 - 19