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Molality

Describes calculations involving molal solutions

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How is molality used?

Molality involves moles: it is the ratio of moles of solute to kilograms of solvent. Molarity, on the other hand, which should not be confused with molality, is the ratio of moles of solute to liters of solution. Colligative (collective) property calculations tend to use molality as opposed molarity.

Credit: Uncle Ariel
Source: http://www.flickr.com/photos/afgeneral/390892712/in/photolist-AxqMw-Cj9Lj-48k7yf-4eo87V-4gGSvk-4ndtoc-4ndttg-4ndtC8-4ndvRD-4ndyb8-4ndKSF-4nhxyh-4nhzM3-4nhzWC-4nhPLW-4pypJd-4uuog6-4v68C9-5iBhEF-5j8KBf-5kjYTX-5GZvmt-5JHzto-5JYCTS-5LCHVh-5LU3P2-5PR3uB-5REc3v-5TM1gQ-5TSUDc-5WdXCk-5WdXCn-5WY8Px-7hoJE5-7kCmGk-7kGebq-7kGeQs-7mqwwc-7muqPN-7mutkd-7mutHY-7mEiwP-7mJb2h-7oDSbV-7qc8vG-7tYCNz-7unfcb-7utnZP-7uPSsY-7v2Yoy-8YEkga/lightbox/
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Antifreeze is important for your car during a snowstorm [Figure1]

Suppose a snowstorm is coming, all the stores are closed, and you need antifreeze for your car's radiator to prevent your car's engine from freezing or splitting. You must make your own antifreeze using materials you have at home. You will need enough soluble antifreeze solution to replace about \begin{align*}\frac{1}{2}\end{align*} of the water in your car's engine. Adding solute to the water decreases the freezing point because the solute molecules physically inhibit the water molecules from creating the uniform structure necessary to form ice crystals. To figure out how much antifreeze needs to be used, you can use molality to calculate the necessary moles of antifreeze per kilogram of water.

Creative Applications

  1. What are the most important differences between molality and molarity? (use the second resource link if necessary)
  2. What are some similarities and differences of the various ways to express concentrations of solutions?
  3. Based upon your knowledge on molality and molarity, which do you believe is more convenient in expressing concentrations of solutions and why?
  4. To help you better understand molality, use the third and fourth resource links to review and test you skills.

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