Question

Determine the concentration of a solution of sodium fluoride (NaF) that has a pH of 8.51 at 25C

Answer 1

I think you're missing some information here.

Answer 2

when you introduce NaF into solution it's going to dissociate: \(NaF\rightarrow Na^+ + F^-\)

After the fluoride anion is going act as a base and abstract protons from water and creates hydroxide anions in the process (this is what causes the pH to increase).
\(F^-+H_2O \rightleftharpoons HF + OH^- \)

so find \([OH^-]\), write an equilibrium expression, use the \(K_b\) to find \([F^-]\), which is equal to [NaF].