Empircial Formulas

1. An empirical formula states the simplest whole number ratio of atoms in a substance.
2. All ionic formulas are empirical (for example we write NaCl instead of Na₂Cl₂ or Na₃Cl₃) and some molecular formulas are empirical.
3. For example:
   1. Aluminum sulfide (ionic) is Al₂S₃ and is empirical.
   2. Water (molecular) is H₂O and is empirical.
   3. Benzene (molecular) is C₆H₆ which is not empirical. Its empirical formula is CH.
4. If we have a sample of a substance and can determine the mass of each element present in the sample then we can determine its empirical formula.
5. To understand how to do this, let's take water for example. In one mole of water we have 2.016g of H and 16.00g of O. If we convert each of these masses into moles we get the familiar ratio: 2.000 moles of H to 1.000 mole of O. See calculation below:
6. If we only had one quarter of a mole of water then the H mass would be 0.504g and the O mass would be 4.00g. If we convert these masses to moles then we get:
7. By simplifying the mole ratio we still get a nice whole number ratio, from which we can construct the correct empirical formula - H₂O:
8. If you had a sample of aluminum chloride that contained 20.23g of aluminum and 79.77g of chlorine, what would be its empirical formula? Click here to see answer below.
9. Given a percent composition you should also be able to calculate the empirical formula. If you know something is 30% carbon and 70% oxygen, just hypothesize a theoretical sample of any arbitrary size and determine what the mass of carbon and oxygen would be. For example, if you had a sample of the above compound with a mass of 100g, then 30g would be carbon and 70g would be oxygen. You can now start the empirical formula calculations.

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