Question

It is recommended that drinking water contain 1.6 ppm fluoride (F⁻) for prevention of tooth decay. Consider a reservoir with a diameter of 4.70 ✕ 10² m and a depth of 14.2 m. (The volume is πr²h, where r is the radius and h is the height.) How many grams of F⁻ should be added to give 1.6 ppm? & Fluoride is provided by hydrogen hexafluorosilicate, H₂SiF₆. How many grams of H₂SiF₆ contain this much F⁻?

Answer 1

Mass of F^- should be added = 3943560g

Mass of H2SiF6 required = 4985054g

Explanation

Radius, r = Diameter/2 = 4.70 ×10²m /2 = 2.35 ×10²m

height , h = 14.2m

Volume of the reservoir = πr²h = 3.143× (2.35 × 10²m)²× 14.2m = 2464724.885m³ = 2464724885L

ppm = mg/L

1.6mg of F^- for 1L

mg of F^- for 2464724885L = (1.6mg/1L)× 2464724885L = 3943559816mg = 3943560g

Conversion factor of F^- mass to H2SiF6 mass = mass of one mole of H2SiF6/mass of F^- in one mole of H2SiF6

= 144.11g/114g/mol

= 1.2641

mass of H2SiF6 required = 1.2641 × 3943560g = 4985054g