The percent composition by mass of an element in a compound is determined by dividing the mass of that element in one mole of the compound by the molar mass of the compound and then multiplying the result by 100%.
Understanding Percent Composition by Mass
Percent composition by mass is a way to describe the relative amounts of each element present in a compound. It represents the percentage of the total mass of the compound that is contributed by each element. This information is crucial in chemistry for various applications, including identifying unknown substances, understanding chemical reactions, and determining empirical and molecular formulas.
Calculating Percent Composition by Mass
Here's a breakdown of the steps involved in determining the percent composition by mass:
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Determine the Molar Mass of the Compound:
- Find the atomic masses of each element in the compound from the periodic table.
- Multiply the atomic mass of each element by the number of atoms of that element in the compound's formula.
- Add up these values to get the molar mass of the compound.
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Calculate the Mass of Each Element in One Mole of the Compound:
- For each element, multiply its atomic mass by the number of atoms of that element present in the compound's formula. This gives the mass of that element in one mole of the compound.
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Calculate the Percent Composition:
- Divide the mass of each element (calculated in step 2) by the molar mass of the compound (calculated in step 1).
- Multiply the result by 100% to express it as a percentage.
Example Calculation
Let's calculate the percent composition by mass of water (H2O):
| Element | Atomic Mass (g/mol) | Number of Atoms | Mass in 1 Mole of H2O (g) |
|---|---|---|---|
| H | 1.008 | 2 | 2.016 |
| O | 16.00 | 1 | 16.00 |
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Molar mass of H2O: 2.016 g/mol + 16.00 g/mol = 18.016 g/mol
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Percent composition of H: (2.016 g / 18.016 g) * 100% = 11.19%
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Percent composition of O: (16.00 g / 18.016 g) * 100% = 88.81%
Therefore, the percent composition by mass of water is approximately 11.19% hydrogen and 88.81% oxygen.
Practical Applications of Percent Composition
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Identifying Unknown Compounds: The percent composition can be used as a "fingerprint" to identify unknown compounds. By comparing the experimental percent composition of an unknown substance to the theoretical percent compositions of known compounds, one can often determine the identity of the unknown.
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Determining Empirical Formulas: The empirical formula of a compound represents the simplest whole-number ratio of the elements present. Percent composition data can be used to determine the empirical formula.
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Stoichiometry: Percent composition is used in stoichiometric calculations to relate the amounts of reactants and products in a chemical reaction.
Summary of Key Insights
- Percent composition by mass indicates the percentage of each element's mass in a compound.
- The calculation involves determining the molar mass of the compound and the mass of each element in one mole of the compound.
- It is a valuable tool in identifying substances, determining formulas, and understanding chemical reactions.
