Determining Molecular Formula

1
Determining Molecular Formula
2
Molecular Formula

• If we know the percent composition of a compound
  in terms of the masses of the elements present we
  can calculate the empirical formula but not the
  molecular formula.
• In order to calculate the molecular formula we
  must know the molar mass.

3
Molecular Formula

• If the empirical formula is not given, follow the
  steps calculate the empirical formula.
• Find the empirical formula mass.
• Compare the empirical formula mass to the molar
  mass.
• molecular formula (empirical formula)n
• n molar mass/empirical formula mass

4
Example Problem 1

• A white powder is analyzed and found to have an
  empirical formula of P2O5. The compound has a
  molar mass of 283.88 g/mol. What is the
  compounds molecular formula?

5
Example Problem 2

• A compound used as an additive for gasoline to
  help prevent engine knock shows the following
  percentage composition 24.27 C, 4.07 H, and
  71.65 Cl. The molar mass is known to be 98.96 g.
  Determine the empirical formula and the molecular
  formula for this compound.

6
Example Problem 3

• A certain hydrocarbon (a compound consisting of
  carbon and hydrogen) is 82.66 carbon by mass and
  has a molar mass of 58.12 g/mol. Determine the
  molecular formula of this hydrocarbon.