Determining a compound's molecular formula involves a straightforward process using the empirical formula and molar mass. The molecular formula represents the actual number of atoms of each element in a molecule, while the empirical formula shows the simplest whole-number ratio of atoms.
Steps to Calculate the Molecular Formula
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Determine the Empirical Formula: This step usually involves finding the mass percentages of each element in the compound and converting them to moles. Then, divide each element's mole number by the smallest mole number to obtain the simplest whole number ratio, representing the empirical formula. For example, if the empirical formula is CH₂O.
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Calculate the Empirical Formula Mass (EFM): Add up the atomic masses of each element according to the empirical formula. For CH₂O, this would be 12.01 (C) + 2 * 1.01 (H) + 16.00 (O) = 30.03 g/mol.
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Determine the Molar Mass (Molecular Mass): The molar mass of the compound must be experimentally determined, often through techniques like mass spectrometry. Let's assume the molar mass is given as 180.18 g/mol for our example.
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Calculate the Ratio: Divide the molar mass by the empirical formula mass: 180.18 g/mol / 30.03 g/mol ≈ 6
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Multiply Subscripts: Multiply the subscripts in the empirical formula by the ratio calculated in step 4. In our example: (CH₂O) * 6 = C₆H₁₂O₆. This is the molecular formula.
Example
Let's say we have a compound with an empirical formula of CH and a molar mass of 78 g/mol.
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EFM of CH: 12.01 (C) + 1.01 (H) = 13.02 g/mol
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Ratio: 78 g/mol / 13.02 g/mol ≈ 6
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Molecular Formula: (CH) * 6 = C₆H₆ (Benzene)
Key Considerations
- The molecular formula is a whole-number multiple of the empirical formula.
- You need both the empirical formula and the molar mass to determine the molecular formula.
- Various analytical techniques can be used to determine the molar mass, including mass spectrometry.
