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Homework Help: How to balance this formula

  1. Dec 16, 2008 #1

    Ca_3(PO_4)_2 + H_3PO_4 -> Ca(H_2PO_4)_2

    i got the same problem with this one
    MgO + H_3PO_4 -> Mg_3(PO_4)_2 + H_2O
    each time i balance one element i get the other which i balance before
    to be dis balanced

    what is the general way of solving such things
    Last edited: Dec 16, 2008
  2. jcsd
  3. Dec 16, 2008 #2


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    I suggest starting with the elements that only occur in one part, usually you end with C, H and O because they occur most.
    For example, for the first one,
    a Ca3(PO4)2) + b H3PO4 -> c Ca (H2 PO4)2

    First start with setting a = 1. You only have Ca in one element on the left, so you need c = 3 to get the amount of Ca on the right correctly. Then you have also fixed an amount of PO4 on both sides, and you will have to use b to compensate that. Then you have used all your freedom, so the H should automatically be balanced (if not, the reaction is not possible). Finally, check if you have fractions somewhere, and multiply through by some number to get integers everywhere.

    If you are more mathematically inclined, you can write down a set of equations:
    For Ca: 3a = c
    For PO4: 2a + b = 2
    For H: 3b = 4c
    and solve that. Afterwards, again multiply a, b and c by some number to get rid of possible fractions (e.g. if you find a = 1, b = 1/2 and c = 1/5 multiply by 10 and use a = 10, b = 5, c = 2).
  4. Dec 16, 2008 #3


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  5. Mar 20, 2009 #4
    i cant see how you get this equations..
    why you put po4 as one instead doing for "p" ad oxiden
    why in the second one you have only 2
  6. Mar 20, 2009 #5


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    transgalactic, CompuChip is trying to generalize your first example and used a , b, and c as counting variables (or coefficients?) for each compound.

    Myself, starting in your Ca example, I would start with the Ca atoms first. The compound on the leftside has 3 Ca atoms, so you probably want 3 Ca atoms on the rightside. How many calcium dihydrogenphosphates would you need on the rightside? You would best pick 3 of these. Next, account for the phosphorus atoms. Do you have the same number of atoms of phosphorus on both sides? If not, then you have another adjustment to make. WORK WITH IT!
  7. Mar 20, 2009 #6


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    Becasue they are always combined in the same way. You will have two linearly dependent equations - no new information, more places to make a mistake.

    Must be a typo, shoud be 2c.
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