1. Chemical Formula of the Compound: This tells you the types of elements present and their respective ratios in the compound. For example, the chemical formula of water is H₂O, indicating that it contains two hydrogen atoms and one oxygen atom.
2. Atomic Masses of the Elements: You need to know the atomic mass of each element present in the compound. These values can be found on the periodic table.
Here's how to calculate the percent composition:
1. Calculate the molar mass of the compound:
* Multiply the atomic mass of each element by its subscript in the chemical formula.
* Add the results together.
2. Calculate the mass of each element in the compound:
* Multiply the atomic mass of each element by its subscript in the chemical formula.
3. Calculate the percent composition of each element:
* Divide the mass of each element by the molar mass of the compound.
* Multiply by 100 to express the answer as a percentage.
Example: Calculate the percent composition of water (H₂O)
1. Molar mass of H₂O:
* H: 1.01 g/mol x 2 = 2.02 g/mol
* O: 16.00 g/mol x 1 = 16.00 g/mol
* Total: 2.02 g/mol + 16.00 g/mol = 18.02 g/mol
2. Mass of each element:
* H: 1.01 g/mol x 2 = 2.02 g/mol
* O: 16.00 g/mol x 1 = 16.00 g/mol
3. Percent composition:
* H: (2.02 g/mol / 18.02 g/mol) x 100 = 11.21%
* O: (16.00 g/mol / 18.02 g/mol) x 100 = 88.79%
Therefore, water is composed of approximately 11.21% hydrogen and 88.79% oxygen by mass.
