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# Calculating Molar Mass

## Describes calculations for determining molar mass from changes in boiling or freezing point.

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Practice Calculating Molar Mass
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What Does It Weigh?

### What Does It Weigh?

Credit: PD-USGO
Source: https://commons.wikimedia.org/wiki/File:LC_MS_pic.jpg

We are all familiar with the textbook problem: determine the molecular formula for a compound from its percent composition. We obtain the percent composition experimentally, then use the molecular weight of the compound to ascertain the actual formula. But where do we get that molecular weight? If we don’t know what the compound is, how do we determine this important piece of information?

#### News You Can Use

• There are a number of techniques available for the measurement of molecular weight. All of them are based on the assumption that we are working with a pure compound. What can throw this assumption off? If there are impurities (small amounts of other compounds) our measurements may be in error. If there is water present, that will affect our final calculations. So we need to start with a very pure compound, and that takes a lot of preparatory work.
• Assume that we now have our pure compound. One way to determine the molecular weight is to use a mass spectrometer. This device bombards a sample with electrons to form ions. The ions are separated in a magnetic field and the ionized products are detected. This information provides a reliable molecular weight and is especially useful for small compounds.
• Large molecular-weight materials (proteins, synthetic polymers) require more sophisticated techniques. The amount of light scattered by a polymer solution is widely used, as is separation by column chromatography. Other approaches employ complex electrophoresis techniques and mass spectroscopy.
• Credit: Iwan Gabovitch
Source: http://www.flickr.com/photos/qubodup/11585511555/

Gel electrophoresis allows scientists to see the different sizes of DNA pieces in a sample. Each blue line represents DNA pieces of a certain size [Figure2]

• Watch a video explaining the principle of mass spectrometry at the link below: https://www.youtube.com/watch?v=tOGM2gOHKPc

#### Show What You Know

1. What happens when electrons bombard a molecule?
2. Who originated the technique of electrospray ionization for biomolecules?
3. Why do we need to use average weights when studying polymers?
4. Why are proteins treated with sodium dodecyl sulfate before electrophoresis?
5. How does polyacrylamide gel electrophoresis separate proteins?

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