Let’s use those mole conversions!



Empirical formula tells you the lowest ratio of
atoms within a molecule
Molecular formula tells you the actual ratio of
atoms within a molecule
The empirical formula CAN equal the
molecular formula, but it doesn’t have to.
Example:
A compound contains 39.99% carbon, 6.73%
hydrogen, and 53.28% oxygen. The molar mass
is 180.18 g/mol. What is the empirical and
molecular formulas for this compound?

Step 1:
◦ Convert grams to moles
 (If the problem gives you percents, assume that you
have 100g of sample and use the percents as grams)
C:
39.99𝑔
1
H:
6.73𝑔
1
O:
53.29𝑔
1
×
×
1 𝑚𝑜𝑙
12.01𝑔
1 𝑚𝑜𝑙
1.01𝑔
×
= 3.33 𝑚𝑜𝑙
= 6.66 𝑚𝑜𝑙
1 𝑚𝑜𝑙
16.00𝑔
= 3.33 𝑚𝑜𝑙

Step 2:
◦ Divide all of the moles from Step 1 by the lowest
number of moles from Step 1.
C:
39.99𝑔
1
H:
6.73𝑔
1
O:
53.29𝑔
1
×
×
1 𝑚𝑜𝑙
12.01𝑔
1 𝑚𝑜𝑙
1.01𝑔
×
= 3.33 𝑚𝑜𝑙
= 6.66 𝑚𝑜𝑙
1 𝑚𝑜𝑙
16.00𝑔
= 3.33 𝑚𝑜𝑙
Lowest #

Step 2:
◦ Divide all of the moles from Step 1 by the lowest
number of moles from Step 1.
C:
39.99𝑔
1
H:
6.73𝑔
1
O:
53.29𝑔
1
×
×
1 𝑚𝑜𝑙
12.01𝑔
1 𝑚𝑜𝑙
1.01𝑔
×
=
=
1 𝑚𝑜𝑙
16.00𝑔
3.33 𝑚𝑜𝑙
3.33 𝑚𝑜𝑙
6.66 𝑚𝑜𝑙
3.33 𝑚𝑜𝑙
=
=1
=2
3.33 𝑚𝑜𝑙
3.33 𝑚𝑜𝑙
=1
These numbers tell you how
many of each atom you have
in the molecule.
 Sometimes
you may end up with
non-whole numbers in Step 2. If
this happens:
◦ multiply all of the moles by 2 if any end in .5
◦ multiply all of the moles by 3 if any end in .3

Step 3:
◦ Write the formula using the numbers found in Step
2.
C:
39.99𝑔
1
H:
6.73𝑔
1
O:
53.29𝑔
1
×
×
1 𝑚𝑜𝑙
12.01𝑔
1 𝑚𝑜𝑙
1.01𝑔
×
=
=
1 𝑚𝑜𝑙
16.00𝑔
3.33 𝑚𝑜𝑙
3.33 𝑚𝑜𝑙
6.66 𝑚𝑜𝑙
3.33 𝑚𝑜𝑙
=
=1
=2
3.33 𝑚𝑜𝑙
3.33 𝑚𝑜𝑙
=1
This is the
empirical formula:
CH2O

If the problems asks for molecular formula,
move on to Step 4:
◦ Find the molar mass of the empirical formula and
divide the GIVEN molar mass by this number.
Molar mass of empirical formula:
CH2O
12.01g + 2*1.01g + 16.00g = 30.03 g/mol
GIVEN molar mass = 180.18 g/mol
180.18 𝑔/𝑚𝑜𝑙
=6
30.03𝑔/𝑚𝑜𝑙
multiplier

Step 5:
◦ Multiply the number of atoms in the empirical
formula by the multiplier.
180.18 𝑔/𝑚𝑜𝑙
=6
30.03𝑔/𝑚𝑜𝑙
multiplier
(CH2O) * 6 =
C6H12O6
This is the molecular formula.