Fluoridation of city water supplies has been practiced in the United States for

Question

Fluoridation of city water supplies has been practiced in the United States for several decades. It is done By continuously adding sodium fluoride to water as it comes from a reservoir. Assume you live in a medium-sized city of 150,000 people and that 660 L (170 gal) of water is used per person per day. What mass of sodium fluoride (in kilograms) must be added to the water supply each year (365 days) to have the required fluoride concentration of 1 ppm (part per million) "that is, 1 kilogram of fluoride per 1 million kilograms of water? (Sodium fluoride is 45.0% fluoride, and water has a density of 1.00 g/cm^3.)

Explanation / Answer

Solution :-

Lets find the total volume of the water

(150000 people * 660 L / 1 person)*(365 days / 1 year) = 3.61*10^10 L

Now lets convert volume of water to mass

3.61*10^10 L * 1 kg/L = 3.61*10^10 kg water

Now lets calculate the mass of the fluoride needed to have desired concentration of 1 ppm

3.61*10^10 kg water * 1 kg fluoride / 1*10^6 kg water = 36100 kg fluoride

Now using the percentage of the fluoride lets calculate the mass of sodium fluoride using the percentage

36100 kg * 100 % / 45.0 % = 8.022*10^4 kg NaF

So the mass of the NaF is 8.022*10^4 kg

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