Past Exam Question

[andy456]

I'm having trouble figuring out this question. Any help would be appreciated

Question 4
a. In order to help prevent tooth decay, fluoride ions at a level of 0.9 mg L–1 of F– are added to Melbourne’s public water supplies. The fluoride ions are obtained by adding sodium fluoride (NaF) to the water.

i. Calculate the mass of sodium fluoride in mg that must be present in one litre of water to produce a concentration of fluoride ions of 0.90 mg L–1.

ii. What mass of sodium fluoride, in kilogram, must be added to a 750 ML reservoir (1 ML = 106 L) to produce a concentration of fluoride ions of 0.90 mg L–1?

iii. Calculate the number of fluoride ions swallowed by a person who drank one litre of water from the reservoir.

[vexx]

^
hope this is right:

well for i)
0.9mg=0.0009g
so c(in molL-1)=(0.0009gL-1/M(NaF))
and then using that concentration, times by 1.00 L, to get mol.
then using mol, times by molar mass to get mass.

ii) again, c(in molL-1) must be 0.0009gL-1/M(NaF)
so find mol= (750x106)x ^molL-1
and then x molar mass of NaF, and divide by 1,000 to get mass in kg.