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Homework Help: Need Help with Chemistry!

  1. May 11, 2008 #1
    Just answer me thinking that I don't know anything.

    I have a few problems.

    1. How do I know how many ions does a compound break into by just looking at the chemical formula of the compound?

    2. If two compounds are mixed, how do I know which ions will combine and precipitate?

    3. Most importantly, if the chemical formula is written in text, how do I know how many atoms of each element are present. For e.g. if Copper (II) Chloride is written, how do I figure out that it is CuCl?

    Please help me.
  2. jcsd
  3. May 11, 2008 #2
    3.Cooper(II) Chloride is CuCl2

    It` s already written Copper(II), that means you have copper with oxidation number +2
    Chlorine element is HALOGEN, the same like eg. fluorine, bromine, iodine. They all have oxidation number -1.
    Now when you know oxidation number of both elements, you just easily exchange them and all what you have to do it`s to write them behind their neighbour element.
    Oxidation number + 2 from copper you write behind chlorine and oxidation number -1 from chlorine you write behind copper ( if the oxidation number is one you don`t have to write it ).Then you look at formula CuCl2 and you will see that there is 1 copper atom and 2 chlorine atoms.
  4. May 11, 2008 #3
    OK, thanks,

    what about the other questions
  5. May 12, 2008 #4
    2. If two compounds are mixed, how do I know which ions will combine and precipitate?

    Solubility table and I believe it's table 4.1 in Brown, LeMay and Bursten, The Central Science 10th edition.
    Most of the time : AB + CD --> AD + BC . I usually use exchange method and see any precipitation.
  6. May 19, 2008 #5
    i need help!


    1. Calculate the volume of carbon dioxide at 273 K and 1.01 * 10^5 Pa which would be produced when 1.25g of calcium carbonate reacts completely with HCL.


    2. When 41.18 cm^3 of a solution of silver ions with concentration of .2040 mol dm^-3 is added to a solution of XO4 -3 ions 1.172g of precipitate are formed.
    i) calculate the amount in moles of Ag+ used in the reaction.
    ii)Calculate molar mass of precipitate
    iii)Determine the relative atomic mass of X and identify the element.

    thanks for the help even if you can't answer it! thanks!
  7. May 20, 2008 #6
    This requires practice with common cations and anions. Ionizable molecules are usually, if not always, polar. The acid HCl, for example, has a partially positive pole (H) and a partially negative pole (Cl). In solutions, HCl will ionize to form H+ and Cl-. HNO3 will produce H+ and NO3-.

    For substances such as KMnO4, the polarity is not as easily apparent, but it can still be deduced that this molecule will produce K+ and MnO4-.

    These anions (Cl-, KMnO4-, NO3-) and may other common ones can be found on a special page of almost any chemistry textbook.
  8. May 20, 2008 #7


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Hello soccergirl,

    It's best to start a new thread when you have a new question.

    Also, if you can explain how far you got and include what equations you think are useful, you're more likely to get help.
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