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Homework Help: Converting parts per million (ppm) and mol of ions

  1. Jun 2, 2010 #1
    1. Fluorine compounds are added to water, the concentration being 0.90 ppm.

    a. What amount of fluoride present 1.0g of water?

    Solution- (just need to know what the number mean)

    Divided 0.90 pm by 1,000,000, which I assume is the grams of substance per kilogram?

    Then divided that number by fluorines molar mass (19) to get the answer of 4.7x10^-8

    b. How much fluoride ions would you swallow if you drank 200ml water?

    2. Convert 0.00036M (molarity) of Ca(OH)2 to ppm

    Solution- I found the molar mass of the compound being ~74

    3. For 0.20M of K2SO4, calculate mol of K+ ions, Sulfate ions and oxygen atoms

    Solution - Molar mass I found to be ~174

    4. Why should antacid tables be chewed before being swallowed?

    5. Find concentration of vitamin c in g L^-1 when 40mg/100ml and do same for carbonhydrate at 8.5g/100ml

    Solution- I'm guessing I just divided 40mg/100ml and 8.5g/0.1L but not sure if this makes sense?
    Last edited: Jun 2, 2010
  2. jcsd
  3. Jun 2, 2010 #2
    Any help would be appreciated...
  4. Jun 2, 2010 #3


    Staff: Mentor

    For #1, ppm means parts per million. If fluoride is added to water to make a concentration of .9 ppm, that means that there are .9g of fluoride per million grams of water. The amount of fluoride in 1 g. of water would be .9/1,000,000 * 1 g.

    For #4, what would be the effect of chewing up antacid tablets (not tables) before swallowing them?
  5. Jun 4, 2010 #4
    Hey thanks!

    Could you help me with the 200ml question? I figured out all the rest
  6. Jun 4, 2010 #5


    Staff: Mentor

    I'm not sure what you mean by "how much fluoride ions." This is ambiguous, as it could mean the actual number of flouride ions or it could be the volume. If you're interested in the volume, the concentration of fluoride ion is .9/1,000,000, so multiply that ratio by the volume of water, 200 ml, to get the volume of fluoride ion.
  7. Jun 4, 2010 #6
    I think it means the number of ions, the answer at the back is 5.7 times 10^18
  8. Jun 4, 2010 #7


    Staff: Mentor

    From something you said in the first post, the .9 PPM is the concentration by mass. PPM is NOT the number of grams per kg. That would be parts per thousand.

    1 PPM would be 1 microgram (one millionth of a gram) of whatever in a gram of the substance it's in.

    When you did problem 1a you had .9/1,000,000 * 1 g * 1 mole/19 g = 4.74 x 10-8 moles of Fluoride.

    To get the number of molecules, multiply the number of moles by Avogadro's number, which IIRC is 6.022 x 1023.

    That will give you the number of Fl molecules in 1 g. of fluoridated water. Take that number an multiply by the number of grams of water in 200 ml.

    It's been a very long while since I've done these calculations, so I'm a little rusty, but I don't think I'm steering you wrong.

    As a tip, be sure to put units in - otherwise it's very easy to get an answer that's meaningless.
  9. Jun 5, 2010 #8


    User Avatar

    Staff: Mentor

    Volume? No, it is either number of moles or number of ions.

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