Question

Flouride is known to prevent tooth decay so it is added to the city water supply for drinking. A medium sized city is populated with 150,000 people. Each person uses approximately 660.0 L of water each day. How many kg of NaF must be added to the water supply each year to have the required flouride concentration of 1.0 ppm?

Answer 1

3.6 × 10^4 kg NaF

Step 1. Calculate the daily volume of water used

Volume = 150 000 persons × (660.0 L/1 person) = 9.90 × 10^7 L

Step 2. Calculate the daily mass of the water

Mass of water = 9.90 × 10^7 L water× (1.00 kg water water/1 L water)

= 9.90 × 10^7 kg water

Step 3. Calculate the daily mass of NaF

Daily mass of NaF = 9.90 × 10^7 kg water × (1.0 kg NaF/10^6 kg water)

= 99.0 kg NaF

Step 4. Calculate the yearly mass of NaF

Yearly mass of NaF = (99.0 kg NaF/1 day) × (365.25 days/1 yr)

= 3.6 × 10^4 kg NaF