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Date:_______________ Period:____
Chemistry Honors
Empirical and Molecular Formulas
Empirical and Molecular Formulas
An empirical formula of a compound shows the smallest whole-number ratio of the atoms in the
compound. Empirical formulas can be interpreted at the atomic (small -scale) and at the molar (largescale) level. They may or may not be the same as the molecular formula of the compound.
A molecular formula indicates the actual number of each kind of atom present in a molecule of the
compound.
Determining the Empirical Formula of a Compound
Sample Problem 10.11 (Note: Each step needed to solve the problem is shown here to show you the
problem solving process that is followed.)
A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical
formula of the compound?
Step 1: Analyze the information provided.
Known:
% N = 25.9
% O = 74.1
Unknown:
Empirical formula: N ?O?
Step 2: Solve for the unknown
-
-
Because percent means parts per 100, you can assume that 100.0 g of the compound contains
25.9 g N and 74.1 g O.
Use these values to convert to moles.
1 𝑚𝑜𝑙 𝑁
25.9 𝑔 𝑁 ×
= 1.85 𝑚𝑜𝑙 𝑁
14.01 𝑔 𝑁
1 𝑚𝑜𝑙 𝑂
74.1 𝑔 𝑂 ×
= 4.63 𝑚𝑜𝑙 𝑂
16.00 𝑔 𝑂
The lowest ratio of N to O is N 1.85O4.63 however this is NOT the lowest WHOLE number ratio!
Therefore we need to divide both quantities (the molar quantities) that were just calculated by
the smallest number calculated.
1.85 𝑚𝑜𝑙 𝑁
= 1 𝑚𝑜𝑙 𝑁
1.85
4.63 𝑚𝑜𝑙 𝑂
= 2.50 𝑚𝑜𝑙 𝑂
1.85
-
The ratio of N to O is now N 1 O2.5. This is still not a whole a number ratio. How can we get both
subscripts to equal whole numbers……………you guessed it! We must multiply each part of the
ratio by the smallest whole number that will convert both subscripts to whole numbers.
1 𝑚𝑜𝑙 𝑁 × 2 = 2 𝑚𝑜𝑙 𝑁
2.5 𝑚𝑜𝑙 𝑂 × 2 = 5 𝑚𝑜𝑙 𝑂
-WHEWW! We have finally reached the lowest whole number ratio of N to O possible!
N 2 O5
Does this answer make sense?
 In the space below summarize the steps needed to determine the empirical formula of a
compound.
 What if the percent composition was not given to you? What would the first step of the process
be then?
Complete the practice problems on the following page.
Empirical Formula Practice Problems (Be sure to follow the problem solving steps and your summarized
steps for determining empirical formula.)
1. Calculate the empirical formula of a compound that is 67.6% Hg, 10.8% S, and 21.6% O.
2. 1,6-diaminohexane is used to make nylon. What is the empirical formula of this compound if it is
62.1% C, 13.8% H, and 24.1 % N?
3. If 8.87 grams of phosphorus react with 11.43 grams of oxygen, what is the empirical formula fo
the compound formed? (Hint: Think carefully about the information that you are given and
where you can go from here. Dimensional analysis is actually your friend! Do you need to
determine percent composition here?)
4. A small coil of magnesium ribbon is placed in a crucible. The crucible and cover have a mass of
21.35 grams. Re-measuring the mass, including the magnesium, gives 21.63 grams. The crucible,
cover, and contents are heated and then cooled and treated with water. A second heating and
measuring of the final product (magnesium combined with oxygen) gives a mass of 21.82 grams.
Determine the empirical formula of this oxide of magnesium.
Molecular Formulas
The molecular formula of a compound can be the same as its empirical formula or it can be a whole
number multiple of the empirical formula. In order to determine the molecular formula of a compound
from its empirical formula you will need the empirical formula mass (efm) and the molar mass of the
compound as well.
Determining the Molecular Formula of a Compound
Sample Problem 10.12
Calculate the molecular formula of a compound whose molar mass is 60.0 g/mol and empirical formula
is CH4N.
Step 1: Analyze the information given.
Known:
Empirical formula: CH4 N
Molar mass = 60.0 g/mol
Unknown:
Molecular formula
Step 2: Solve for the unknown
-
Calculate the empirical formula mass (efm) 30.06 g/mol
-
Divide the molar mass by the efm
(2)
(a nswer must be a whole number; no decimals a t this s tep!)
-
Multiply the empirical formula subscripts by the whole number calculated.
Complete the practice problems on the next page.
Molecular Formula Practice Problems
1. Find the molecular formula of ethylene glycol, which is used as antifreeze. The molar mass is 62
g/mol and the empirical formula is CH3O.
2. Which pair of molecules has the same empirical formula?
a. C2 H4O2 and C6 H12O6
b. NaCrO4 and Na2Cr2O7
3. The compound methyl butanoate smells like apples. Its percent composition is 58.8% C, 9.8% H,
and 31.4% O and its molar mass is 102 g/mol. What is its empirical formula? What is its
molecular formula? (How could you check your final answer for this problem?)
4. The formula mass of a compound is 92.0 g. Analysis of the compound shows that there are
0.608 grams of nitrogen and 1.388 grams of oxygen. What is the molecular formula of this
compound?
5. The mass of the CoCl 2· xH2O was measured before and after being dehydrated. The initial mass
was 1.62 grams and the final mass was 0.88 grams. What is the empirical formula for this
compound?