Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

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


    User Avatar
    Science Advisor
    Homework Helper

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook