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Finding pH given molarity and volume

  1. May 10, 2009 #1
    1. The problem statement, all variables and given/known data
    If 50ml of 0.004M H2CrO4 was combined with 50 ml of 0.005M HNO3 what is the pH of the resulting solution?


    2. Relevant equations
    Moles H3O=Moles OH ?
    Formula for KA



    3. The attempt at a solution
    So, here's what I have so far.

    I made the H2CrO4 represent the acid, and the HNO3 represent the base. I then subtracted them to get the excess, and then found the molarity of that. But I have no idea if I chose the right things to represent acids and bases, in fact I have no idea if I'm doing the right thing. Somebody please help!
     
  2. jcsd
  3. May 10, 2009 #2

    Redbelly98

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    Both of these are acids, there is no base here.

    Also, do these substances dissociate completely, or must Ka be taken into account?

    p.s. Welcome to Physics Forums :smile:
     
    Last edited: May 10, 2009
  4. May 10, 2009 #3
    I believe HNO3 is a strong acid- you can assume that dissociates completely. However, the other acid is NOT a strong acid--you will need Ka for that.

    BUT. I'm assuming since the Ka is very small, adding the H+ ions from that species will not change the # of H+ you get from HNO3 because of sig figs.

    Therefore, .05L x .005M H+ (from the complete dissociation of HNO3) = 0.00025mol H+

    next, find the molarity. 100ml total = .1L
    .0025 Molar H+
    -log .0025 = 2.6 = pH
     
    Last edited: May 10, 2009
  5. May 10, 2009 #4

    Borek

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    No.

    No.

    No.
     
  6. May 10, 2009 #5
    so sad... haha
     
  7. May 10, 2009 #6
    Hell of a thread here.
     
  8. May 10, 2009 #7

    Redbelly98

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    Look up Ka for H2CrO4, and set up the equilibrium equation. That is the key here.
     
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