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Empirical Formula

How to write simplified molecular formulas

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How Many Are There?

How Many Are There?

Credit: Dr Tr
Source: http://www.flickr.com/photos/dr_tr/6847272774/
License: CC BY-NC 3.0

And we're goin' to Surf City, 'cause it's two to one

You know we're goin' to Surf City, gonna have some fun

You know we're goin' to Surf City, 'cause it's two to one

You know we're goin' to Surf City, gonna have some fun, now

Two girls for every boy

- Jan and Dean (1963)

The chorus to this 1963 hit single for Jan and Dean spells out every teen-age boy’s wish – more girls than boys present at the beach (it is usually the other way around). Only the ratio is indicated, not the exact number of people. So we don’t know how big this crowd really was and how successful the guys would be at meeting girls.

Why It Matters

  • You go to a large party at school or work. About all you can tell easily is how many men and how many women there are. You count 20 men (M) and 15 women (W). So your empirical formula is M20W15 or M4W3. You don’t know who belongs to what family or what relationships exist until you gather further information (how many husbands? wives? families? parents? grandparents?). If you had that data, you could then write a more detailed formula for the group.
  • Credit: UNE Photos
    Source: http://www.flickr.com/photos/unephotos/6831875658/
    License: CC BY-NC 3.0

    What's the ratio of men to women in this photo? [Figure2]


  • When a new molecule is discovered or synthesized, we don’t know a lot about it. We may have some idea of the different atoms involved because we used them to make the compound (and we can’t just “lose” atoms). But some of the atoms may not have reacted (that is usually the brown sludge in the bottom of the reaction flask). Some of the atoms may have been lost into the atmosphere as gases. The rest are most likely in the new molecule, but we don’t know which ones or how many.
  • This molecule must be highly purified before a successful analysis can be carried out. Impurities that might be present will definitely skew the results and give false information. Contamination with a solvent such as ether will increase the fractions of carbon and hydrogen, providing misleading data. For organic compounds, analyses for carbon, hydrogen, and nitrogen are commonly performed. Special measurements must be made if halogens or sulfur are suspected.
  • Once the elemental composition is determined, we need to then find the molecular weight. There are some classic techniques that involve measuring changes in freezing point or changes in vapor pressure. Acids can be titrated to obtain data for calculations. Modern physical techniques involve fragmenting the material and separating the fragments for further direct analysis.

Show What You Know

Use the links below to learn more about molecular and empirical formulas. Then answer the following questions.

  1. What does the empirical formula tell us?
  2. What is the first step in determining the formula for a molecule?
  3. How are hydrogen and carbon determined?
  4. How is nitrogen measured?
  5. What technique can measure molecular weight directly?

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Image Attributions

  1. [1]^ Credit: Dr Tr; Source: http://www.flickr.com/photos/dr_tr/6847272774/; License: CC BY-NC 3.0
  2. [2]^ Credit: UNE Photos; Source: http://www.flickr.com/photos/unephotos/6831875658/; License: CC BY-NC 3.0

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