MOLECULAR
FORMULAS
(here you will be using empirical to help
you determine molecular formulas)
Example 1: Find the molecular formula of a
compound with empirical formula CH2 and a
molar mass of 84.18g/mol.
STEP 1: Find the empirical molar mass.
Example 1: Find the molecular formula of a
compound with empirical formula CH2 and a
molar mass of 84.18g/mol.
STEP 2: Divide the molecular mass (GIVEN) by
the empirical formula mass to get a mass
multiple.
Example 1: Find the molecular formula of a
compound with empirical formula CH2 and a
molar mass of 84.18g/mol.
STEP 3: Use the mass multiple to determine
the molecular formula – multiply the
empirical formula by the mass multiple.
FINDING MOLECULAR
FORMULAS...
...using percentage composition data.
Example 2: Determine the molecular formula of vitamin C
(ascorbic acid). It contains 40.5% C, 4.6% H, and 54.5% O.
Molar mass = 176.14g/mol.
***Assume 100g of substance***
STEP 1: Find the amount of each in moles.
Example 2: Determine the molecular formula of vitamin C
(ascorbic acid). It contains 40.5% C, 4.6% H, and 54.5% O.
Molar mass = 176.14g/mol.
STEP 2: Determine the simplest ratio.
Example 2: Determine the molecular formula of vitamin C
(ascorbic acid). It contains 40.5% C, 4.6% H, and 54.5% O.
Molar mass = 176.14g/mol.
STEP 3: Find the lowest whole number ratio to
determine the empirical formula.
Example 2: Determine the molecular formula of vitamin C
(ascorbic acid). It contains 40.5% C, 4.6% H, and 54.5% O.
Molar mass = 176.14g/mol.
STEP 4: Determine the molar mass of C3H4O3.
Example 2: Determine the molecular formula of vitamin C
(ascorbic acid). It contains 40.5% C, 4.6% H, and 54.5% O.
Molar mass = 176.14g/mol.
STEP 4: Determine the molar mass of C3H4O3.
Example 2: Determine the molecular formula of vitamin C
(ascorbic acid). It contains 40.5% C, 4.6% H, and 54.5% O.
Molar mass = 176.14g/mol.
STEP 5&6: Solve for the mass multiple, then
multiply the empirical formula by x.
HOMEWORK: