How many fluoride ions would you swallow if you drank a 200 mL glass of Melbourne water? (Recall 1 mL of water weights 1 g.)

Fluorine compounds are added to Melbourne’s supplies of drinking water to give a concentration of fluoride ions of about 0.90 ppm (1 ppm = 1 g in 10^6 g).

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Here, you'll have to see how many ions per mL of water, then multpily by how many mL of water you have.

Your ppm measurement gives you the concentration you need. Here is another way of expressing ppm (keep in mind, 1g fluoride per 10^6 g H2O is effectively dividing the quantities as we do here):

1 ppm Fluoride = 1 g fluoride/10^6 g H2O

Because we have 0.90 ppm fluoride, we effectively have multiplied both sides of the above equation by 0.90:

0.9 * 1 ppm Fluoride = 0.9 * 1 g fluoride/10^6 g H2O

This gives us a way to convert from ppm to grams of fluoride:

0.9 ppm Fluoride = 0.9 g fluoride/10^6 g H2O

Now, we figure out how many grams of fluoride we have by noticing that the above units allow us to convert from grams of water to grams of fluoride. This way we can take our 200 mL of water (200g because of the density 1 g/mL water given in the problem) and convert to grams of fluoride in the method show below:

Total g Fluoride = Total g H2O * 0.9 g Fluoride/10^6 g H2O

Total g Fluoride = 200 * 0.9/10^6 = 1.8 * 10^-4 g Fluoride

Now, we have our total grams of fluoride. Now, we can convert grams of fluoride to moles of fluoride. Remember, we want to count ions, which is exactly what finding the number of moles allows us to do. The way we convert to moles is by dividing the number of grams of fluoride by the molar mass of fluoride (which for simplicity's sake, we'll say is equivalent to the molar mass of fluorine = 19.0 g/mol found on the period table (and the link below)):

Moles Fluoride = Total g Fluoride / Molar Mass Fluoride

Moles Fluoride = 1.8 * 10^-4 g / 19.0 g/mol

Moles Fluoride = 9.4 * 10^-6 mol

Now, recall from stoichiometry, we can convert moles to number of ions simply by multiplying by Avogadro's Number (6.02 * 10^23 units/mole):

Ions Fluoride = Moles Fluoride * Avogadro's Number

Ions Fluoride = 9.4*10^-6 mol * 6.02 * 10^23 ions/mole

**Ions Fluoride = 5.7 * 10^18 ions**

And there you have it! Just to recap, use the known concentration of fluoride to find your mass of fluoride, then use the mass and the periodic table to find moles, then use the number of moles to find the number of ions!

**Sources:**

concentration of fluoride ions of about 0.90 ppm

and 1 ppm = 1 g in 10^6 g

that is the concentration of floride ions wold be

0.90 * (1/10^6) that is 9.0*10^-7 g

so it would be 9.0*10^-7g of floride per 1 g of water

Now density of water is 1g/mL and the volume of water is 200mL

Denisty = mass/volume

Mass = density * volume

Mass of water = 1g/mL * 200 mL = 200 g

So the amount of floride ions would be

200 g water * (9.0*10^-7 g floride ion/ 1 gram of water)

1.8*10^-4 g of floride ions.

So if 200 mL of water is consumed, then there would be 1.8*10^-4 g of floride ions

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