Question

A particular water sample that is saturated in CaF2 has a Ca2+ content of 115 ppm (that is, 115 g Ca2+ per 10^6g of water sample)
What is the F− ion content of the water in ppm?

Answer 1

I would calculate this using atomic masses:

M(Ca) = 40.08 and M(F) = 19
Because there are two F, try proportion 38/40.08 * 115 ppm.

Answer 2

First of all in my opinion 115ppm of CaF2 means 115g of CaF2 in 1000000 g of sample. As far as Ca++ ions are concerned, there shall be 8.88*10^23 ions of Ca++ and there number of flouride ions will be twice i.e. 1.78*10^24 F- ions in 115 ppm CaF2 sample
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