Transcript Empirical and Molecular Formulas

Empirical and Molecular
Formulas
Empirical Formula
Empirical vs. Molecular Formulas



Molecular Formula – a formula that specifies
the actual number of atoms of each element in
one molecule of a compound.
Empirical Formula – a formula with the
smallest whole-number mole ratio of the
elements that make up a compound.
Empirical Formula

Empirical Formula


May or may not be the same as the molecular
formula
Molecular formula is always a simple multiple of
the empirical formula
 ex. H2O2


Empirical formula is HO
Molecular formula is TWO times the empirical
formula

How to calculate an empirical formula
How to calculate:
STEP 1: You will be given either masses or
percent composition.
 STEP 2: If you are given % composition,
turn it into grams by assuming a 100.0 g
sample. NOTE: If you are given mass, you
do not need to do this step.
 STEP 3: Convert the masses to the number
of moles of each element.



STEP 4: Figure out the proportion of moles of
each element in the compound by dividing
each by the smallest number of moles.
STEP 5: If step 4 resulted in whole numbers,
you are done! However, if there were
decimals, you will need to multiply by small,
whole numbers until you have whole numbers.
An example:

STEP 1:


STEP 2:


Compound is 40.05% S and 59.95% O
I assume 100 g of the compound, so it is:
 40.05 g S and 59.95 g O
STEP 3:


40.05 g S•(1 mol S/32.07 g S) = 1.249 mol S
59.95 g O•(1 mol O/16.00 g O) = 3.747 mol O
Continued…

STEP 4:




STEP 5:


1.249 mol S : 3.747 mol O
Divide each by 1.249 (smallest number in ratio)
1 mol S : 3 mol O
SO3
You are done! The compound is sulfur
trioxide.
A way to remember those steps:





A Poem by Mr. Erlenbeck:
Percent to mass
Mass to mole
Divide by small
Multiply ‘til whole
Molecular Formula


Molecular Formula – this tells us how many
atoms of each type there really are in the
compound.
Can two substances have the same empirical
formula but be different?


YES! Benzene vs. acetylene: C6H6 vs. C2H2
What is their empirical formula? How is this
different from ionic compounds?
Calculating Molecular Formula

STEP 1:


STEP 2:


Calculate the empirical mass (mass of the
empirical formula).
STEP 3:


You will be given the molar mass of the compound
and the empirical formula.
Divide the given molar mass by the empirical
mass. You should get a small whole number.
STEP 4:

Multiply the subscripts of the empirical formula
with the number obtained.
Molecular Formula Example

STEP 1:


STEP 2:


The empirical mass is 12.01g + 2*1.01g + 16.00g
= 30.03 g
STEP 3:


The empirical formula is CH2O and the molar
mass is 180.18 g.
180.18 g/ 30.03 g = 6
STEP 4:

CH2O becomes C6H12O6


Closure:
Acetylene is a gas that is used as a fuel for
welding. Benzene is a liquid solvent.




How are they similar?
How are they different?
Why are they different?
Why is one a gas at room temperature and one a
liquid?