Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Water Chemistry Problem

  1. Sep 17, 2010 #1
    The question:

    Nitrate concentrations exceeding 44.3 mg NO3-/L are a concern in drinking water due to the infant disease, methemoglobinemia.

    Nitrate concentrations near three rural wells were reported as 0.01 mg NO3- N/L, 1.3 mg NO3- N/L, and 20.0mg NO3- N/L. Do any of these three wells exceed the 44.3 ppm level?

    The problem:

    I have no problem with conversions, it's just the units in this case. I've asked peers about the NO3- N/L unit and they said the N stands for nitrogen. If this is so, I don't know what the implications are. Whats the difference between the NO3-/L and NO3- N/L unit?
     
  2. jcsd
  3. Sep 17, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    Doesn't make sense to me.

    I can easily imagine NO3- reported as 0.01 mg N/L, but the way you have it written it is - to say the least - strange.
     
  4. Sep 17, 2010 #3

    alxm

    User Avatar
    Science Advisor

    Hmm, I've https://www.physicsforums.com/showpost.php?p=2162757&postcount=2"..

    But to elaborate a bit more; you have the whole nitrogen cycle, so nitrogen in water (with living stuff in it) is being converted between nitrate, nitrite, ammonia, etc.
    Therefore they distinguish between "N/L" nitrogen per liter (the mass of N from all compounds) "NO3- N/L", nitrate nitrogen per liter (just the nitrate nitrogen) and nitrate/L (the mass of nitrate, including oxygen).
     
    Last edited by a moderator: Apr 25, 2017
  5. Sep 17, 2010 #4
    But for every mole of NO3-, there is 1 mole of N... So is there no conversions to be made here? I can just simply look at the values and if they are less than 44.3, they don't exceed the limit?
     
  6. Sep 17, 2010 #5

    Borek

    User Avatar

    Staff: Mentor

    ppm is usually weight/weight, so you need some conversion.
     
  7. Sep 17, 2010 #6
    But a mg/L is a ppm, at least concerning aqueous solutions.

    there is 10^6 mg of water in a litre.

    I think I'm still missing something though. It can't be as easy as just looking at the given values...
     
    Last edited: Sep 17, 2010
  8. Sep 17, 2010 #7

    alxm

    User Avatar
    Science Advisor

    Your units are in mg/L, not mol/L. One mole of N doesn't weigh the same as 1 mole of NO3-
     
  9. Sep 18, 2010 #8
    Ah that is a good point, it's starting to become a little clearer.

    So:

    (0.01 mg NO3- N/L) x (1 mol N / 14000 mg N) x (1 mol NO3- / 1 mol N) x (62000 mg NO3- / 1 mol NO3-)

    = 0.04 mg NO3-/L

    Am I understanding this concept correctly?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook