MASS PERCENT AND
EMPIRICAL
FORMULA
I. Mass Percent
Also known as percent composition.
1.  Determine the number of moles of
each element in one mole of a
compound.
2.  Multiply by the molar mass of the
element.
3.  Divide by the molar mass of the
compound.
 
I. Mass Percent
 
 
Ex #1: What is the mass percent of
each element in N2O5?
Ex #2: What is the mass percent of
each element in C3H8O?
II. Empirical Formula
 
The simplest mole ratio of elements
in a compound.
II. Empirical Formula
1. 
2. 
3. 
4. 
5. 
Experimentally determine mass
percents (given).
Assume 100 g of compound.
Convert mass to moles.
Divide by smallest number.
Determine simplest whole
number ratio.
II. Empirical Formula
 
 
Ex #3: A compound contains
38.67% C, 16.22 % H, and 45.11%
N by mass. What is its empirical
formula?
Ex #4: A compound contains
44.88% K, 18.40 % S, and 36.72%
O by mass. What is its empirical
formula?
III. Molecular Formula
Find empirical formula.
2.  Divide molar mass (given) by
empirical formula mass.
3.  Multiply empirical formula by
this factor.
1. 
III. Molecular Formula
 
Ex #5: A compound is 39.97% C,
13.41% H, and 46.62% N by mass.
Its molar mass is 60.10 g/mol.
What is its molecular formula?