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Empirical Formula

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Empirical Formula

Antoine Lavoisier's laboratory

What might the scientists in this picture be discussing?

When the French scientist Antoine Lavoisier conducted his experiments, he did not know what the products of reactions were going to be.  He had to isolate the material (whether he was heating mercury or measuring gases from breathing) and then study its elemental composition before he could understand the processes that were occurring.

Discovering that a new compound exists is the start of a long research project.  In order to make this new compound in the lab, we need to know a lot about its structure.  Often, the place to start is to determine the elements in the material.  Then we can find out the relative amounts of each element to continue our evaluation of this new material.

Empirical Formula

An empirical formula is a formula that shows the elements in a compound in their lowest whole-number ratio. Glucose is an important simple sugar that cells use as their primary source of energy. Its molecular formula is C 6 H 12 O 6 . Since each of the subscripts is divisible by 6, the empirical formula for glucose is CH 2 O. When chemists analyze an unknown compound, often the first step is to determine its empirical formula. There are a great many compounds whose molecular and empirical formulas are the same. If the molecular formula cannot be simplified into a smaller whole-number ratio, as in the case of H 2 O or P 2 O 5 , then the empirical formula is also the molecular formula.

How would we determine an empirical formula for a compound? Let’s take a compound composed of carbon, hydrogen, and oxygen.  We can analyze the relative amounts of each element in the compound.  When we get a percent figure for each element, we now know how many grams of each are in 100 grams of the original material.  This allows us to determine the number of moles for each element.  The ratios can then be reduced to small whole numbers to give the empirical formula.  If we wanted a molecular formula, we would need to determine the molecular weight of the compound.

Summary

  • The empirical formula tells the lowest whole-number ratio of elements in a compound.
  • The empirical formula does not show the actual number of atoms.

Practice

Questions

Use the link below to answer the following questions:

http://pages.towson.edu/ladon/empiric.html

  1. How is an empirical formula calculated?
  2. How is the number of moles determined from the percent composition?

Review

Questions

  1. Define “empirical formula.”
  2. Why is C 6 H 12 O 6 not considered to be an empirical formula for glucose?
  3. Can the empirical formula for a compound be the same as the molecular formula?
  4. What do we need to know in order to determine a molecular formula from an empirical formula?
  5. Give three examples of compounds whose empirical formulas are the same as their molecular formulas.

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