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Test tomorrow need help

  1. Dec 13, 2006 #1
    I'm taking chemistry in my sophmore year, I need help with writing names or chemical formulas, to be honest I don't know anything whatsoever.

    For example: HOW can I possibly know that Calcium Selenide would be CaSe in chemical formula

    Or that (NH4)2SO4 is ammonium sulfate ??????????

    I really need help!
  2. jcsd
  3. Dec 13, 2006 #2


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    To be honest, if you don't know "anything whatsoever," then I doubt anyone can teach you, from scratch, about chemical formulae in one day! However, I'll try the way I was taught when first coming across these sorts of problems.

    In the case of calcium selenide: The Calcium ion has a charge 2+, and the selenium ion 2-. The molecule must have no net charge, and so we can see that these charges will cancel out, and the correct formula must be CaSe.

    In the case of ammoium sulphate: The charge on the ammonium ion (NH4) is 1+, and the charge on the sulphate ion (SO4) is 2-. So, in order to form a molecule with no net charge we must have 2 ammonium ions to one sulphate ion, hence the formula (NH4)2SO4.
  4. Dec 13, 2006 #3


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    You need to learn (i.e., memorize, either directly, or through the periodic table) the valencies of the common species (elements and radicals). This is something that typically takes several days or weeks worth of familiarization.

  5. Dec 13, 2006 #4


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    And if you start to get tired studying all that chemistry, here is a chemistry humor page from the website that Gokul linked to:

  6. Dec 14, 2006 #5
    Thanks guys, I failed the test...lol :S
  7. Dec 14, 2006 #6


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    Staff: Mentor

    Oh well. At least you know to study more next time, eh? Hang in there.
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