Gravimetric and volumetric

[Studyinghard]

Fluoride ions at a level of 0.9mgL^-1 of F- are added to Melbourne's public water. Fluoride ions obtained by adding Sodium fluoride.

a) calculate the mass of sodium fluoride in milligrams that must be pressent in 1L of water to produce a concentration of fluoride ions of
0.90mgL^-1.

[shivesh.varmaxD]

i don't post on these boards enough, don't have the time..

anyway, here we go. you have the concentration of fluoride (in a mass/volume form), and the volume of fluoride. from this, you can work out the mass of fluoride present in 1L of water (given in the concentration really, but for sake of completeness i'll include that in the calc.), then calculate the amount of fluoride, translate this to the amount of sodium fluoride, then calculate the mass of sodium fluoride.

so in 1L of water you need: c(F-) = 0.90mg/L

m(F-) = c * V = 0.90mg/L * 1L = 0.90mg = 0.0009g

n(F-) = m / M = 0.0009 / 19 = 0.00004797mol

n(NaF) = n(F-) = 0.00004797mol  (as there is one fluoride ion for every sodium fluoride formula)

m(NaF) = n * M = 0.00004797mol * (23 + 19) = 0.00198947g = 1.98947mg (to however many sig figs the question requires)

hope i didnt make any careless errors there. similar simple calculations like this are used throughout grav and volumetric analysis.

good luck!