CHEMISTRY
November 1st, 2011
Brainteaser 11/1/11
• A compound is 30.4% Nitrogen and 69.6%
Oxygen. Determine the empirical formula
Molecular Formula
• Molecular Formula-actual number of
elements in the formula
– Can be determined following the same steps
as used for empirical formula
– The empirical formula of the compound
must be known first!!
– Once the empirical formula of the compound
has been determined, calculate the molar
mass of the empirical formula and compare it
to the actual molar mass (given).
Empirical vs. Molecular
• Empirical:
– Like knowing you have 3 bags of apples, but
you don’t know how many apples you have in
each bag.
• Molecular:
– Knowing that each apple is 1 gm, and the
weight of all the bags combines is 9 grams.
– How many apples are in each bag??
Practice Problem
• What is the molecular formula of a
compound with a percent composition by
mass of 65.2% Arsenic and 34.8% Oxygen
and has a molar mass of 460 grams?
Practice Problem
• A blue solid is found to contain 36.84%
Nitrogen and 63.16% Oxygen, what is the
empirical formula of this compound?
Molecular formula
• Molecular Formulas
– To determine the molecular formula of a compound
Experimentally determined molar mass = multiple of smallest whole
Mass of empirical formula
number ratio
• For example:
– Molar mass of acetylene is 26.04 g/mol and mass of the
empirical formula, CH, is 13.02 g/mol
• This tells you that the molecular formula is 2X that of the empirical
formula.
Molecular formula: (CH)2 = C2H2
Molecular Formula
• Molecular Formula-actual number of
elements in the formula
– Can be determined following the same steps
as used for empirical formula
– The empirical formula of the compound must
be known
– Once the empirical formula of the compound
has been determined, calculate the molar
mass of the empirical formula and compare it
to the actual molar mass (given).
Practice Problem
• What is the molecular formula of a
compound with a percent composition by
mass of 65.2% Arsenic and 34.8% Oxygen
and has a molar mass of 460 grams?
• A blue solid is found to contain 36.84%
Nitrogen and 63.16% Oxygen, what is the
empirical formula of this compound?
Molecular formula
• Molecular Formulas
– To determine the molecular formula of a compound
Experimentally determined molar mass = multiple of smallest whole
Mass of empirical formula
number ratio
• For example:
– Molar mass of acetylene is 26.04 g/mol and mass of the
empirical formula, CH, is 13.02 g/mol
• This tells you that the molecular formula is 2X that of the empirical
formula.
Molecular formula: (CH)2 = C2H2