The question should be: "How do you find the empirical formula of a compound, given the mass or percentage composition of elements, as you might obtain from a mass spectrometer?"
Here's how to determine the empirical formula of a compound, given the mass or percentage composition of its elements:
Determining the empirical formula from mass spectrometry data involves several steps. While mass spectrometry itself doesn't directly give the empirical formula, it provides information (mass-to-charge ratios of ions) that can help determine the elemental composition, which is necessary for finding the empirical formula. The process generally involves these steps:
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Obtain Mass or Percentage Composition: If a mass spectrometer directly provides information on elemental composition or the mass of elements in a molecule or fragment, you will need to get the mass or percentage composition for each element. This initial step is crucial and involves translating data obtained from mass spectrometry (ion peak intensity, mass-to-charge ratios, etc.) to the mass composition of each element.
- For example, if the mass spectrum analysis shows a fragment containing carbon and hydrogen with a mass ratio of 12:1, it implies there’s 12 g of Carbon to every 1 g of Hydrogen.
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Convert Mass to Moles: Once you know the mass or percentage of each element, convert these values into moles. To do this, divide the mass of each element by its respective molar mass (atomic weight from the periodic table).
- For example, using the previous fragment example:
- Moles of Carbon (C) = 12 g / 12 g/mol = 1 mol
- Moles of Hydrogen (H) = 1 g / 1 g/mol = 1 mol
- For example, using the previous fragment example:
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Determine the Smallest Whole-Number Ratio: Divide the number of moles of each element by the smallest number of moles of all elements present. This will give you a ratio of moles. You will likely need to multiply those numbers until you get a whole number ratio. This ratio represents the relative number of atoms of each element in the compound, which is the empirical formula.
- Continuing the example from before:
- Carbon (C): 1 mol / 1 mol (smallest number) = 1
- Hydrogen (H): 1 mol / 1 mol (smallest number) = 1
- Therefore the empirical formula for this fragment is CH.
- Continuing the example from before:
Summary in Table Format
| Step | Description | Example |
|---|---|---|
| 1 | Obtain Mass or Percentage Composition of each element. | Mass spec or other analyses provides that there is 12 g of Carbon and 1 g of Hydrogen |
| 2 | Convert mass to moles by dividing by the element's molar mass | Carbon: 12 g / 12 g/mol = 1 mol; Hydrogen: 1 g / 1 g/mol = 1 mol |
| 3 | Determine the simplest whole number mole ratio by dividing by the smallest amount of moles | Carbon: 1/1 = 1; Hydrogen 1/1 = 1; Empirical Formula = CH |
It's important to remember that the empirical formula shows the simplest ratio of atoms, not necessarily the actual number of atoms in a molecule. For that, you would need the molecular formula, which requires more information, such as the molar mass of the molecule, often determined through mass spectrometry as well.
