Fluoride, in the form of sodium silicofluoride (Na 2 SiF 6 ), is commonly added
ID: 955977 • Letter: F
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.
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.
Explanation / Answer
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