Name: ____________________________________Per: ______
Steps to Find Empirical Formulas
1) Convert given into moles -(if given % - assume a 100g sample, then use molar mass
to convert grams into moles)
2) Divide each by the least number of moles to get the mole ratio for each element in
compound.
3) If necessary, get the whole number ratio by multiplying each number by the same
number.
.2 x 5
.25 x 4
.33 x 3
.5 x 2
.67 x 3
.75 x 4
.80 x 5
4) Write empirical formula putting elements in order they are given in problem.
To Find the Molecular Formula
Given
Molar Mass of Molecular Formula
Molar Mass of Empirical Formula
= Whole Number Ratio
Calculated
Use the whole number ratio and multiply the subscripts of the empirical formula to
find the molecular formula.
Tips:
1. Always have the bigger number over the smaller number.
2. If get a decimal that is too far away from a whole number, go back and find
mistake.
Emperical & Molecular Example
Emperical Formula
Given: A compound contains 84.12% carbon and 15.89% hydrogen. What is the empirical formula of this
compound?
mol
84.12 g C 
 7.004 mol C
Step 1- assume the % is out of
12.01g
100g & convert grams to moles:
mol
15.89 g H 
 15.76 mol H
1.008 g
Step 2- divide by the lowest
number of moles (divide each
by the same number):
(C )
7.004
 1.000
7.004
(H )
15.76
 2.250 (not close enough to a whole number to round )
7.004
Step 3- get a whole number ratio (you
can’t have 2.25 atoms of H).
(Remember, whatever you do to one of
the numbers you must do to the others)
Step 4- write empirical formula in order
elements are given in the problem:
(C ) 1.000  4  4.000
( H ) 2.250  4  9.000
empirical formula  C H
4 9
Molecular Formula
Given: The empirical formula we found was C4H9. If the molar mass of the compound is 114.22g/mol, what
is the molecular formula of the compound?
Step 1- calculate the molar mass of the empirical formula:
C 4 H 9 (molar mass) : (12.01  4)  (1.008  9)  57.11g / mol
Step 2- divide the given molar mass of the molecular formula by the calculated molar mass of the
empirical formula.
Given Molar Mass (Molecular Formula)
114.22

 2.000
Calculated Molar Mass (Empirical Formula)
57.11
Step 3- multiply the subscripts in the empirical formula by the number calculated in Step 2.
C4 H 9
4 2  8
9  2  18
molecular formula  C H
8 18
Name: _____________________________________Per: _______________ Date: __________
Empirical and Molecular Formulas Intro
1) What is the empirical formula of a compound that is 78.0% iron and 22.0% oxygen?
2) What is the empirical formula of a compound that is 32.37% sodium, 22.58% sulfur, and 45.05% oxygen?
3) What is the empirical formula of a compound that is 56.58% potassium, 8.69% carbon, and 34.73%
oxygen?
4) If a compound is found to be 70.00% iron and 30.00% oxygen, what is the empirical formula?
5) Find the empirical formula if a sample contains 58.96% Na and 41.04% O.
6) Find the empirical & molecular formulas of a compound that is 40.0% sulfur and 60.0% oxygen;
the molar mass is 240.2g/mol.
7) Find the empirical and molecular formulas of a compound that is 25.94% nitrogen and 74.06%
oxygen; the molar mass is 324.0g/mol.
8) A compound is 75.46% carbon, 4.43% hydrogen, and 20.10% oxygen. It has a molecular
weight of 318.31 g/mol. What are the empirical and molecular formulas for this compound?
9) A sample contains 4.783g phosphorus and 5.717g oxygen when analyzed. Find the empirical and
molecular formulas if the molar mass of the compound is 204.9g.
10) Find the empirical formula if a sample contains 42.9% carbon and 57.1% oxygen.
11) A compound is 57.1% C, 38.1% S, and 4.8% H. What are its empirical and molecular formulas, if the
molar mass is 168.3 g/mol?
12) Find the empirical formula if a sample contains 72.2% Mg and 27.8% N.
13) Find the empirical & molecular formulas of a compound that is 92.26% carbon and 7.74% hydrogen; the
molar mass is 78 g/mole.
14) Find the empirical and molecular formulas of a compound that contains 40.0% C, 6.6 %H, and 53.4% O;
the molar mass is 120. g/mole.
15) A sample is 0.43g hydrogen & 6.93 grams oxygen. The molecular mass of the compound is 34.0 g/mol.
What are the empirical and molecular formulas of the compound?
16) What is the empirical formula of a compound that is 42.9% carbon, and 57.1% oxygen?
17) What is the empirical formula of a compound that is 52.7% potassium and 47.3% chlorine?
18) What is the empirical formula of a compound that is 75% carbon and 25% hydrogen?
19) What are the empirical and molecular formulas for a compound with 35.33% carbon, 5.08% hydrogen,
and 59.59% chlorine; it has a molecular weight of 475.9 g/mol?
20) What are the empirical and molecular formulas for a compound with 45.01% carbon, 7.55% hydrogen and
47.44% fluorine, and it has a molecular weight of 400.3 g/mol