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How do you find out formulas?

  1. Sep 16, 2009 #1
    So its the beginning of chemistry and I don't know how to make formulas. BUT its very important I learn now otherwise I will have a very hard year...

    For ex:
    It says, "Fill in the correct formulas for the given anion and cation."


    SO2-4 and Al3+ (how do i combine those two to make a formula)


    Another ex:
    Al3+ and Cr2O2-7
    PLEASE EXPLAIN HOW TO DO THESE.

    Thanks so much for any help:)!
     
  2. jcsd
  3. Sep 16, 2009 #2

    mgb_phys

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    The resulting molecule must be uncharged.
    So if you have Al(3+) and SO4(2-) you need to find how many 3+ you need to combine with how many 2- to get zero.
    This is just a least common multiple elementary school maths problem.
    In this case the LCM is 6, so 2*(3+) and 3*(2-) =0 giving Al2(SO4)3
     
  4. Sep 17, 2009 #3
    Write out the cation and the anion with their charges. For your second example, this would be written as Al[tex]^{3+}[/tex]Cr[tex]_{2}[/tex]O[tex]_{7}[/tex][tex]^{2-}[/tex]. Then write the charge of the cation as the subscript of the anion, and vice versa, ignoring the plus and minus of course. It then looks like Al[tex]_{2}[/tex](Cr[tex]_{2}[/tex]O[tex]_{7}[/tex])[tex]_{3}[/tex]. This is basically what mgb phys said, only it's a quicker (and many people starting chemistry find it easier) way to do it. It's also important to note that if you have charges like 4+ and 2-, you must simplify it to 2 and 1 when you write the subscripts.

    EDIT: When I wrote the subscripts they came up as superscripts for some reason. Hopefully you can still tell what I meant.
     
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