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A quick PPM question

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data

    Calculate PPM of *Fluoride* in standard solution. NaF (dissolved in solution) = 1.0534g in 0.250L distilled water.

    2. Relevant equations

    PPM = Mol/L * g/mol * 1000

    3. The attempt at a solution

    C=n/V, n = m/M

    n = 1.0534/41.99 = 0.02509
    c = n/V = 0.02509/0.250 = 0.1M

    PPM = 0.1M * 41.99g/mol * 1000 = 4213.59PPM

    BUT! Sure that's the PPM of NaF in the solution, but to get the PPM of FLUORIDE do I need to divide it by 2? (NaF, 1:1 ratio) or do I use the mW of F in the PPM equation? (19 instead of 41.99)?

    Thanks for any help.
  2. jcsd
  3. Oct 30, 2008 #2


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    Staff: Mentor

    Dividing by two doesn't make sense - 0.1M solution of NaF is 0.1M in both Na+ and F-.
  4. Oct 31, 2008 #3
    Oh right! So the PPM of Fluoride still will be 4213.59?

    So it's like saying out of 50 cars you'll still only have 50 steering wheels?

    Do my calculations look ok?

  5. Oct 31, 2008 #4


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    Staff: Mentor

    No. ppm is mass per mass and mass of F- is different from mass of NaF.

    Yes, but it all depends on the question asked. If you are asked - how many steering wheels in 50 cars, there is 1 steering wheel in each car, so there are 50 steering wheels. But if you are asked - what part of the mass of 50 cars are their steering wheels - you cant say half of the mass of the cars (even if there are 50 cars and 50 steering wheels), because steering wheel is much lighter than the car. Same with NaF - 50 atoms of F per 50 molecules of NaF, but F is not half of the mass.
  6. Oct 31, 2008 #5
    Ah yeah I get what you mean. The question is asking to calculate the concentration of Fluoride in mg/L (PPM).

    I'm guessing this would lean to where the steering wheels mass is much less than the cars mass? So I need to use 19 (MW of F-) in the PPM equation?

    Since it's a 0.1M conc. of NaF (essentially 0.1M conc. of F also)?
  7. Oct 31, 2008 #6


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    Staff: Mentor

    Looks like you got it :smile:
  8. Oct 31, 2008 #7

    Just another thing, the answer came to be 1900ppm in 0.250L. I had to dilute it so that I took 0.01L from that solution and made it up to 0.1L. To find the ppm of the diluted solution is it as simple as dividing the ppm by 25? Because I took 1/25 of the solution?

    I'm not sure since PPM is mg/L, so it must be related to the volume somehow.

    Dividing 0.1M by 25 brings me to 0.004M and putting this into the PPM equation gives me 76ppm.
    Last edited: Oct 31, 2008
  9. Oct 31, 2008 #8


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    Staff: Mentor

    No. But you already know that.
  10. Oct 31, 2008 #9
    Lol thanks Borek :embarrassed:
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