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 630 L (160 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/cm3

7.67 x 10⁵ kg sodium fluoride must be added to the water supply each year

Explanation:

First, let's calculate the amount of water the city consumes per year:

150000 people * 630 l / people * day * 365 days = 3.45 * 10¹⁰ l

Now let's calculate the amount of fluoride to be added, knowing that in

1.00 x 10⁶ l there must be 1 kg fluoride. Then, in 3.45 * 10¹⁰ l there must be:

3.45 x 10¹⁰ l * 1 kg fluoride / 1.00 x 10⁶ l = 3.45 x 10⁴ kg fluoride

This amount is 45 % of the amount of sodium fluoride, then, the amount of sodium fluoride to be added is:

100 % * 3.45 x 10⁴ kg sodium fluoride / 45.0 % = 7.67 x 10⁵ kg sodium fluoride.