Question

Water is fluoridated to prevent tooth decay. (a) How many liters of 1.0 M H2SiF6 should be added to a reservoir with a diameter of 100 m and a depth of 20 m to give 1.2 ppm F2? (1 mol H2SiF6 provides 6 mol F.) (b) How many grams of solid H2SiF6 should be added to the same reservoir to give 1.2 ppm F2?

The answers are A: 1.7 x 10^3 L and B: 2.4 x 10^5 g

I just need to know how to get them.

Answer 1

a) area of the reservoir = 3.14 x (100 m / 2)^2 x 20 m

= 1.57 x 10^5 m3

= 1.57 x 10^8 L ( 1m3 = 10^3 dm3 = 10^3 L )

and

mass of F2 in reservoir = 1.2 mg /L x 1.57 x 10^8 L

= 1.884 x 10^5 g

and

moles of F2 = 1.884 x 10^5 g / 37.997 g/mol = 4.96 x 10^3 mol

therefore

and moles of  H2SiF6 = 4.96 x 10^3 mol / 3 = 1.65 x 10^3 mol

and

volume of 1.0 M H2SiF6 requires =  1.65 x 10^3 L

= 1.7 x 10^3 L

b) the mass of solid H2SiF6 = 1.65 x 10^3 mol x 144.09 g/mol

= 2.38 x 10^5 g

= 2.4 x 10^5 grams