Question

Fluoride, in the form of sodium silicofluoride (Na₂SiF₆), is commonly added to municipal drinking water to prevent tooth decay. It is generally added at a level of 1.0 mg/L, but its maximum concentration level (MCL) cannot exceed 4.0 mg/L. You have been assigned to prepare a liter of a standard solution that contains 1.000 mg/L fluoride.

Explain, in detail, how you would prepare this solution so a labmate could prepare it from your instructions. Your answer must include a brief explanation of the calculations you did to get the mass of substance that will be used to prepare the solution.

Answer 1

consider 1 L of solution

mass of fluoride = 1 mg = 1 x 10-3 g

now

we know that

moles= mass / molar mass

so

moles of fluoride = 1 x 10-3 / 19 = 5.263 x 10-5

now

consider Na2SiF6

we can see that

moles of Na2SiF6 = ( 1/6) x moles of fluoride

moles of Na2SiF6 = (1/6) x 5.263 x 10-5

moles of Na2SiF6 = 8.772 x 10-6

now

mass = moles x molar mass

so

mass of Na2SiF6 = 8.772 x 10-6 x 188

mass of Na2SiF6 = 1.65 x 10-3 g

mass of Na2SiF6 = 1.65 mg

so

the steps are

1) take 1.65 mg of Na2SiF6

2) add 1L of water

3) the solution contains 1 mg / L fluoride