To determine the percent composition from an empirical formula, you calculate the mass of each element in one mole of the compound, divide by the molar mass of the compound, and then multiply by 100%.
Steps to Calculate Percent Composition from an Empirical Formula:
Here's a breakdown of the steps involved:
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Assume you have one mole of the compound. This simplifies the calculations.
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Determine the mass of each element in one mole of the compound. Multiply the subscript of each element in the empirical formula by its atomic mass (found on the periodic table). This gives you the mass contributed by each element in one mole of the compound.
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Calculate the molar mass of the compound. Add the masses of all the elements calculated in step 2. This is the mass of one mole of the entire compound.
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Calculate the percent composition of each element. Divide the mass of each element (calculated in step 2) by the molar mass of the compound (calculated in step 3) and multiply by 100%.
% element = (mass of element in 1 mole of compound / molar mass of compound) * 100%
Example:
Let's determine the percent composition of water (H₂O).
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Assume 1 mole of H₂O.
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Mass of each element:
- Hydrogen (H): 2 (subscript) * 1.01 g/mol = 2.02 g
- Oxygen (O): 1 (subscript) * 16.00 g/mol = 16.00 g
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Molar mass of H₂O: 2.02 g + 16.00 g = 18.02 g/mol
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Percent composition:
- % H = (2.02 g / 18.02 g) * 100% = 11.21%
- % O = (16.00 g / 18.02 g) * 100% = 88.79%
Therefore, the percent composition of water is approximately 11.21% hydrogen and 88.79% oxygen.
Summary
By following these steps, you can accurately determine the percent composition of a compound from its empirical formula, which represents the relative amounts of each element present in the compound. This process is fundamental to understanding the makeup of chemical substances.
