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Molar Mass and Weight

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    Often times we look at food products in terms of the item and how much it weighs. In chemistry terms, this is called the molar mass. Molar mass can be used as a unit conversion. To relate the idea of molar mass to everyday life is something we do all of the time without even thinking about it. For instance, you have an assortment of candy in a bucket. Three kids come up to you and ask you for some candy. Do you weigh the candy and give the three kids equivalent masses? No. That would be unrealistic and probably a little messy. 

    Instead, we just decide that one candy bar should be given to each kid.

    Kid #1 receives one Snickers bar.
    Kid #2 receives one Milky Way.
    Kid #3 receives one Kit Kat bar.

    They are all happy because they received their OWN candy bar; they don't care that one might weigh a little bit more than the other candy bars.

    To summarize, Snickers, Milky Way, and Kit Kat all represent ONE candy bar, however they have different masses...but that's okay. To relate it even more to chemistry, one mole of any compound is equivalent to one mole of any other compound, they just have a different mass depending on what substance you are talking about; just substitute these words and you are back to our everyday example; 'mole' = 'candy bar' and 'compound' = 'type of candy bar'.

    You can use this idea in reverse, if you know the mass of candy, then the amount of candy bars or 'moles' can be calculated.

    If we knew that 1 cup of cheese = 50. g then we could use it in a math equation, this would be our molar mass.

    \[\dfrac{\text{1.0 cup of cheese}}{\text{50.0 g of cheese}}\]

    Example \(\PageIndex{1}\): Cheese Conversion

    Let's pretend that I have 250. g of cheese that I take out of the refrigerator and I want to know how many cups of cheese I have.

    \[\text{mol}_{cheese} (actual) = \text{250 g of cheese} \times \dfrac{\text{1.0 cup of cheese}}{\text{50.0 g of cheese}} = \text{5 cups of cheese}\nonumber\]

    Conversely, if I want to know how much 7.0 cups of cheese would weigh, I could use the same conversion factor, just flipped upside down.

    \[\text{mass}_{cheese} (actual) = \text{7.0 cups of cheese} \times \dfrac{\text{50 g cheese}}{\text{1.0 cup of cheese}} = \text{350 g of cheese}\nonumber\]

    From ChemPRIME: 2.10: The Molar Mass

    Contributors and Attributions

    This page titled Molar Mass and Weight is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Ed Vitz, John W. Moore, Justin Shorb, Xavier Prat-Resina, Tim Wendorff, & Adam Hahn.

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