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High School Lab: Precipitates and Solubility Rules

  1. Dec 5, 2004 #1
    Hello all

    I have a few questions about Precipitates and Solubility Rules

    (a) How do you write an equation for the dissociation of a salt in a solution?
    (b) How do you write a net ionic equation?
    (c) Which ions tend to form soluble salts? insoluble salts? ( Pb[2+] Na[+], k[+], NH4[+], Ag[+], NO3[-]. OH[-]. Cl[-]. CO3[2-], PO4[3-]

    For example is K2C03 the same as

    K2C03 --> 2K + CO3? How do you know when to put the coefficient in front of the ion?

    It also says to write a net ionic equation for the formation of each precipitate that was formed in the experiment (omit spectator ions)

    K3PO4 + BaCl2 ---> ???

    Also is there any good way to memorize or derive the solubility rules??

    Any help is greatly appreciated.

  2. jcsd
  3. Dec 6, 2004 #2


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    Well, this is somewhat about getting used to writing formulae, but I can say that alkali metal ions (along with ammonium) and their hydroxides and halides (along with a whole bunch of anions) have good solubility in water. Nitrates can almost never be precipitated from solution, so any compound with nitrate anion will be soluble in water.

    However, my approach is a simple one; you can always find a salt which is insoluble in water. For example, qualitative potassium ion determination is done with hexanitrocobaltate(II) ion, to give [itex]K_4[Co(NO_2)_6][/itex] precipitate.

    To write a net ionic equation, look for cross-ionic reactions, for example, barium nitrate gives barium sulfate precipitate with the action of saturated calcium sulfate solution:

    [tex]Ba^{2+}_{(aq)}+2NO_3^-_{(aq)}+Ca^{2+}_{(aq)}+SO_4^{2-}_{(aq)}\longrightarrow BaSO_{4(s)}+Ca^{2+}_{(aq)}+2NO_3^-_{(aq)}[/tex]

    You can see that calcium and nitrate ions have no effect here; so you may not include these in the reaction, as a result, a great simplification may be done with this approach:

    [tex]Ba^{2+}_{(aq)}+SO_4^{2-}_{(aq)}\longrightarrow BaSO_{4(s)}[/tex]

    About potassium carbonate, you can understand that potassium is 1+ charged while carbonate ion is 2-. So, two potassium ions must be nearby to neutralize the negative charge.

    Consider these, and please show your work again.
    Last edited: Dec 6, 2004
  4. Dec 6, 2004 #3
    so AgNO3 +BaCl2 net ionic equation is:

    Ag + 2Cl --> Ag2Cl?
  5. Dec 6, 2004 #4


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    No, silver is 1+, so the formula should be AgCl. Two moles of siilver ions will react with one mole of barium chloride to give two moles of AgCl.
  6. Dec 6, 2004 #5


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    Courtigrad, I have a few suggestions :

    1. Learn what the typical oxidation states of different common species are.
    2. Learn how to determine the formula of a compound, given the elements in it.
    3. Learn how to balance equations.
    4. Go through the chapter on Ionic Equations. This is a very important concept and you'll need it as a basis for any further chemistry, so spend some time learning it properly.
  7. Dec 6, 2004 #6


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