Uploaded August 2025 | Updated September 2026, 2 weeks ago
Percent Composition
Dr. DeBacco
Percent composition
Percent composition tells you the percentage by mass of each element in a compound.
This information is crucial for understanding the chemical identity of a substance and is often used in determining empirical and molecular formulas
1. Find the Molar Mass of the Compound
Add up the atomic masses of all atoms in the chemical formula.
Molar Mass- the sum of masses of all the elements in a formula.
the mass of 1 mole of a compound, 1 mole = 6.02x1023
2. Calculate the Mass of Each Element
Multiply the atomic mass of each element by the number of atoms of that element in the formula.
Ex. What is the molar mass of Fe2(SO4)3?
Fe = 55.85 x 2 = 111.70
S = 32.06 x 3 = 96.18
O = 15.99 x 12 = 191.88
399.76 g / mol Fe2(SO4)3
3. Use the Percent Composition Formula
Percent Composition= Mass of Element / Molar Mass of Compound × 100
Percent Composition Example Steps
Ex. Find the % composition of Cu2S.
1. find the molar mass:
Cu = 63.55 x 2 = 127.10
S = 32.07 x 1 = 32.07
159.2 g/ mol Cu2S
2. 127.1 g Cu x 100 = 79.84% Cu
159.2 g Cu2S
32.07 g S x 100 = 20.14% S
159.2 g Cu2S
Basic Steps are Consistent
The sum of all percent compositions should be close to 100%.
Use atomic masses from the periodic table with at least two decimal places for accuracy.
This method works for any compound, just follow the same formula.
Link to Lecture Slides: drive.google.com/file/d/1Ps0hkxR4Pi0yYVmTCB21rj3flA4D4eQZ/view?usp=drive_link
*Due to the description character limit the full work cited for "Percent Composition" can be viewed at... docs.google.com/document/d/1ZzpfR6sDFvhvblyP3GIagZtm7M_26wqK/edit?usp=drive_link&ouid=104237452697237972847&rtpof=true&sd=true
Percent Composition
Dr. DeBacco
Percent composition
Percent composition tells you the percentage by mass of each element in a compound.
This information is crucial for understanding the chemical identity of a substance and is often used in determining empirical and molecular formulas
1. Find the Molar Mass of the Compound
Add up the atomic masses of all atoms in the chemical formula.
Molar Mass- the sum of masses of all the elements in a formula.
the mass of 1 mole of a compound, 1 mole = 6.02x1023
2. Calculate the Mass of Each Element
Multiply the atomic mass of each element by the number of atoms of that element in the formula.
Ex. What is the molar mass of Fe2(SO4)3?
Fe = 55.85 x 2 = 111.70
S = 32.06 x 3 = 96.18
O = 15.99 x 12 = 191.88
399.76 g / mol Fe2(SO4)3
3. Use the Percent Composition Formula
Percent Composition= Mass of Element / Molar Mass of Compound × 100
Percent Composition Example Steps
Ex. Find the % composition of Cu2S.
1. find the molar mass:
Cu = 63.55 x 2 = 127.10
S = 32.07 x 1 = 32.07
159.2 g/ mol Cu2S
2. 127.1 g Cu x 100 = 79.84% Cu
159.2 g Cu2S
32.07 g S x 100 = 20.14% S
159.2 g Cu2S
Basic Steps are Consistent
The sum of all percent compositions should be close to 100%.
Use atomic masses from the periodic table with at least two decimal places for accuracy.
This method works for any compound, just follow the same formula.
Link to Lecture Slides: drive.google.com/file/d/1Ps0hkxR4Pi0yYVmTCB21rj3flA4D4eQZ/view?usp=drive_link
*Due to the description character limit the full work cited for "Percent Composition" can be viewed at... docs.google.com/document/d/1ZzpfR6sDFvhvblyP3GIagZtm7M_26wqK/edit?usp=drive_link&ouid=104237452697237972847&rtpof=true&sd=true
