To calculate the percentage composition of an element in a compound using relative atomic masses, you need to determine the proportion of the element's mass relative to the total mass of the compound, then express that proportion as a percentage. Here's a step-by-step guide:
Steps to Calculate Percentage Composition
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Determine the Chemical Formula: Identify the chemical formula of the compound you are analyzing (e.g., H₂O, NaCl, CO₂).
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Calculate the Relative Formula Mass (Mr):
- Find the relative atomic mass (Ar) of each element in the compound from the periodic table.
- Multiply the Ar of each element by the number of atoms of that element in the formula.
- Sum these values to obtain the Mr of the compound.
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Calculate the Total Ar of the Element of Interest:
- Identify the element for which you want to find the percentage composition.
- Multiply its Ar by the number of atoms of that element in the formula.
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Calculate the Percentage Composition:
- Divide the total Ar of the element by the Mr of the compound.
- Multiply the result by 100 to express it as a percentage.
Formula
The percentage composition of an element can be calculated using the following formula:
Percentage Composition = ( (Total Ar of the element) / (Mr of the compound) ) * 100
Example
Let's calculate the percentage composition of hydrogen in water (H₂O).
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Chemical Formula: H₂O
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Calculate Mr of H₂O:
- Ar of H = 1
- Ar of O = 16
- Mr of H₂O = (2 * 1) + 16 = 18
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Calculate Total Ar of Hydrogen:
- Number of hydrogen atoms = 2
- Total Ar of H = 2 * 1 = 2
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Calculate Percentage Composition of Hydrogen:
- Percentage of H = (2 / 18) * 100 = 11.11%
Therefore, the percentage composition of hydrogen in water is approximately 11.11%.
Summary
Calculating percentage composition from relative atomic mass involves finding the ratio of the mass of an element to the total mass of the compound, expressed as a percentage. By following these steps, you can accurately determine the composition of any element within a given compound.
