The fluoridation of city water supplies has been practiced in the United States
ID: 576046 • Letter: T
Question
The 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 reservoir. Assume you live in a city of 16,000 people and that 660L (170 gal) of water is consumed per person per day. What mass of sodium fluoride (in kg) must be added to the water supply each year (365 days) to have the required fluoride concentration of 1.2 ppm (part per million), that is, 1.2 kg of fluoride per million kg of water? Sodium fluoride is 45% fluoride and water has a density of 1.00 g/cm3.
Explanation / Answer
160,000 people x 660 L / person day x 365 days / year = 3.8544x10^10 L/year
3.8544x10^10 L x 1000 g/L = 3.8544x10^13 g water
3.8544x10^13 / 10^-3 = 3.8544x10^10 kg of water
3.8544x10^10 kg / 10^6 = 3.8544x10^4 million kg of water
1.2 kg fluoride/10^6 kg x (100 g NaF / 45 gm flouride) = 2.667 kg NaF / 10^6 kg water
3.8544x10^4 x 2.667 = 1.028x10^5 kg of NaF
So the mass of NaF added each year is 1.028×10^5 kg.