Calculating Empirical and Molecular Formulas
What is an Empirical Formula?
• The formula of a compound expressed as the smallest possible whole-number ratio
of subscripts of the elements in the formula.
What is a Molecular Formula?
It is the formula of a compound in which the subscripts give the actual number of each
element in the formula.
Calculating Empirical Formula
• What if a percent composition is given? How do we find the empirical formula?
• Example: Chemical analysis of a liquid shows that it is 40.9% C, 4.58% H, and
54.5% O by mass. Calculate the empirical formula of this substance.
– Step 1: Convert to grams
• Assume you have a 100.00 g sample, and convert percentages to
grams.
– 40.9% Carbon is the same as 40.9 g of C
– 4.58% of Hydrogen is the same as 4.58 g of H
– 54.5% of Oxygen is the same as 54.5 g of O
Step 2: Convert grams to moles (divide the numbers above by the atomic mass of each
element from the periodic table.
Pick the smallest
number. It’s 3.41.
C = 40.9g ÷ 12.01g/mol = 3.41 mol
H = 4.58g ÷ 1.01 g/mol = 4.53 mol
O = 54.5 g ÷ 16.00 g/mol = 3.41 mol
Step 3: Divide each by the smallest decimal (in moles) to get whole numbers. * These
numbers will be the subscripts.
3.41 mol ÷ 3.41 = 1
4.53 mol ÷ 3.41 = 1.33
3.41 mol ÷ 3.41 = 1
x 3
3
4
3
Step 4: Place the numbers on the given
element symbols as subscripts.
All these numbers must be close to a whole
number. If one is not, multiply by the smallest
number possible that will get it close to whole
number and if you do that to one, you do it to all.
C3H4O3 This is the
ANSWER
Calculating Molecular Formulas
Example: The empirical formula for a compound is P2O5. Its experimental molar mass is
284 g/mol. Determine the molecular formula of the compound.
Step 1: Find the molar mass of the empirical formula.
2 mol P x 30.97 g = 61.94 g P
5 mol O x 16.00g = + 80.00 g O
141.94 g/mol
Step 2: Molar mass of a compound divided by the molar mass of empirical formula.
Experimental molar mass of compound
284 g/mol
= 2
Molar mass of empirical formula
141.94 g/mol
Step 3: Multiply subscripts from empirical formula by the answer from step 2.
2(P2O5) = P4O10 This is your answer
Calculating Empirical and Molecular Formulas
What is an Empirical Formula?
• The formula of a compound expressed as the smallest possible whole-number ratio
of subscripts of the elements in the formula.
What is a Molecular Formula?
It is the formula of a compound in which the subscripts give the actual number of each
element in the formula.
Calculating Empirical Formula
• What if a percent composition is given? How do we find the empirical formula?
• Example: Chemical analysis of a liquid shows that it is 40.9% C, 4.58% H, and
54.5% O by mass. Calculate the empirical formula of this substance.
– Step 1: Convert to grams
• Assume you have a 100.00 g sample, and convert percentages to
grams.
– 40.9% Carbon is the same as 40.9 g of C
– 4.58% of Hydrogen is the same as 4.58 g of H
– 54.5% of Oxygen is the same as 54.5 g of O
Step 2: Convert grams to moles (divide the numbers above by the atomic mass of each
element from the periodic table.
Pick the smallest
number. It’s 3.41.
C = 40.9g ÷ 12.01g/mol = 3.41 mol
H = 4.58g ÷ 1.01 g/mol = 4.53 mol
O = 54.5 g ÷ 16.00 g/mol = 3.41 mol
Step 3: Divide each by the smallest decimal (in moles) to get whole numbers. * These
numbers will be the subscripts.
3.41 mol ÷ 3.41 = 1
4.53 mol ÷ 3.41 = 1.33
3.41 mol ÷ 3.41 = 1
x 3
3
4
3
Step 4: Place the numbers on the given
element symbols as subscripts.
All these numbers must be close to a whole
number. If one is not, multiply by the smallest
number possible that will get it close to whole
number and if you do that to one, you do it to all.
C3H4O3 This is the
ANSWER
Calculating Molecular Formulas
Example: The empirical formula for a compound is P2O5. Its experimental molar mass is
284 g/mol. Determine the molecular formula of the compound.
Step 1: Find the molar mass of the empirical formula.
2 mol P x 30.97 g = 61.94 g P
5 mol O x 16.00g = + 80.00 g O
141.94 g/mol
Step 2: Molar mass of a compound divided by the molar mass of empirical formula.
Experimental molar mass of compound
284 g/mol
= 2
Molar mass of empirical formula
141.94 g/mol
Step 3: Multiply subscripts from empirical formula by the answer from step 2.
2(P2O5) = P4O10 This is your answer