# Maintaining fluoride ion concentration

#### corryc

This is the problem:
Fluoridation is done by continuously adding sodium fluoride to water drawn from a reservoir. Assume you are responsible for the fluoridation of your city's water. How many tons of sodium fluoride must you add to the water supply each year (365 days) to maintain a fluoride ion concentration of one part per million (one ton of F- ion per million tons of water)? You live in a city with 150,000 people and each person uses 175 gallons of water per day.

Also given:
1 gallon of water has a mass of 8.34 pounds.
1 pound = 453.6 grams.
1 ton = 2000 pounds.
Sodium fluoride is 45.0% fluoride by mass.

According to the back of my textbook, the answer is 89 tons per year. But I can't figure out how to get that answer.

If anyone could help me by showing me how to work out the problem I would really appreciate it. Thanks!

#### topsquark

Forum Staff
This is the problem:
Fluoridation is done by continuously adding sodium fluoride to water drawn from a reservoir. Assume you are responsible for the fluoridation of your city's water. How many tons of sodium fluoride must you add to the water supply each year (365 days) to maintain a fluoride ion concentration of one part per million (one ton of F- ion per million tons of water)? You live in a city with 150,000 people and each person uses 175 gallons of water per day.

Also given:
1 gallon of water has a mass of 8.34 pounds.
1 pound = 453.6 grams.
1 ton = 2000 pounds.
Sodium fluoride is 45.0% fluoride by mass.

According to the back of my textbook, the answer is 89 tons per year. But I can't figure out how to get that answer.

If anyone could help me by showing me how to work out the problem I would really appreciate it. Thanks!
Given 175 gallons of water per person per year gives you how many pounds of water per year? Assuming that fluoridated water weighs the same as water, how many pounds of fluoride do you need to add to make the correct ratio? (Bear in mind that sodium fluoride has 55.0 % water so you need to factor that in.)

Start small and let us know how far you can get.

-Dan