Assume adding the solid NaF does not change the volume of the solution.
(Ka(HF) = 7.1 × 104)
1 Answer

pH = 3.50 ;
[H+] = 10^3.5
[H+] = 3.16 x 10^4
[H+][F]/[HF] = Ka = 7.1 x 10^4
3.16 x 10^4[F]/0.10 = 7.1 x 10^4
[F] = 0.10(7.1 x 10^4)/(3.16 x 10^4)
[F] = 0.225
0.225 moles F/liter x 250 mL x 1 liter/1000 mL = 0.0562 moles F
0.0562 moles F x 1mole NaF/1mole F x 42 grams NaF/1mole NaF= 2.36 g NaF