What are molecular and
empirical formulas?
An empirical formula is the lowest whole number
ratio of the elements combined to form the
substance.
A molecular formula is the ratio of elements which
form the molecule.
Example:
NO2 and N2O4
NO2 is the empirical formula and N2O4 is the
molecular formula
Calculating Empirical Formulas
Method A
For method A, the problem gives you the mass of each
element
Step 1: convert all element masses to moles using
molecular mass/formula mass
Step 2: select the element with the least number of
moles
Step 3: calculate the molar ratios
Step 4: the numbers from the molar ratios are the
subscripts for the empirical formula
Step 5: IF you have molar ratios of 1.5, 2.5, etc.,
multiple each molar ratio by 2 before establishing
subscripts
A compound was analyzed and found to
contain 13.5 g Ca, 10.8 g O, and 0.675 g H.
What is the empirical formula of the
compound?
Step 1: convert to moles
Step 2: identify the least number of moles
Ca at 0.337 mol
Step 3: Divide by the smallest number of moles
Step 4: CaO2H2 or Ca(OH)2
What is the empirical formula of a compound if
a 50.0 g sample of it contains 9.1 g Na, 20.6 g Cr,
and 22.2 g O?
Method B
For problems with percentages:
Step 1: assume you had a starting mass of 100g and
convert all percentages to grams
Step 2: convert all element masses to moles using
molecular mass/ gram formula mass
Step 3: select the element with the least number of
moles
Step 4: calculate the molar ratios
Step 5: the numbers from the molar ratios are the
subscripts for the empirical formula
Step 6: IF you have molar ratios of 1.5, 2.5, etc., multiple
each molar ratio by 2 before establishing subscripts
NutraSweet is 57.14% C, 6.16% H, 9.52% N, and
27.18% O. Calculate the empirical formula of
NutraSweet.
Step 1: convert % to grams
57.14% => 57.14 g C
6.16% H => 6.16 g H
9.52% N => 9.52 g N
27.18% O => 27.18 g O
Step 2: convert to moles
Step 3: ID lowest moles
It is Nitrogen
Step 4: perform molar ratios
Step 5: write the empirical formula
C7H9NO2.5
Step 6: multiply by 2
C14H18N2O5
Determine the empirical formula of the
compound containing 37.5% C, 12.5% H, and
50.0% O by weight.
Determining Molecular Formulas
After you have found the empirical formula, the
problem would need to give you a molecular
mass.
You would find the gram formula mass for your
empirical formula first.
Then you divide the two numbers.
The result is the ratio by which you need to
multiply the empirical formula subscripts.
Problem: The empirical formula is C3H7, and the
molecular weight 86 g/mole. What is the
molecular formula?
C = 12.01* 3 = 36.03g/mol
H = 1.01 * 7 = 7.07 g/mol
43.10 g/mol
86g mol-1/ 43.10g mol-1 = 2
C6H14
Practice
The empirical formula is CH, and the molecular
weight 26 g/mole. What is the molecular
formula?