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Math Help - pH equivalence

  1. #1
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    pH equivalence

    50mL of 0.10mol/L hydrobromic acid is titrated with 0.10mol/l aqueous ammonia. Determine the pH at equivalence.

    NH4+H2O -->NH3+H3O

    n=c x v
    =0.10mol/L x 0.05L
    =0.005mol

    V=0.005mol
    0.1mol/L
    =0.05L

    Vtotal=0.1L

    [NH4Br]=0.005mol
    0.1L
    =0.05mol/L

    NH4 + H2O --> NH3 + H3O
    0.05 0 0
    -x +x +x
    0.05-x x x

    Ka= Kw
    1.8 x10^-5
    =5.6x10^-10= x^2/0.05
    x=5.27x10^-6

    pH=5.28

    I understand everything but how do u know that the Vtotal is 0.1L???
    Last edited by johett; February 3rd 2008 at 08:04 AM. Reason: can read more clearly
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  2. #2
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    make it easy for me to help you,

    firstly what is your questions?
    and can you at try and make your working a little more clear. I can make out that "n = c x v" is means "moles is concentration times volume" but i cant follow the rest of it.
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  3. #3
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    sorry about the first post everyone...I hope the one I have now is a bit easier to understand.
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  4. #4
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    the stiochemistry gives that away.
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  5. #5
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    also, how do u know that you're suppose to use the 0.1L and not 0.05L from the previous step to answer the step after the Vtotal?
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  6. #6
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    you need to look at the stiochemistry of the overall reaction, at equivalence you need to have one mole of the titre for ever mole of the substance your titrating. because they are both of the same concentration you will need the exact same volume of each to get the same number of mole of both of them.

    in simple terms 0.05 + 0.05 = 0.1

    had the concentration of the ammonia been 0.05mol/L you would have needed twice the volume hence the total volume in that case would be 0.05 + 0.1 = 0.15.

    are you following ?
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