The reaction is in equilibrium (like almost every other process). It's easier to write the reaction like this:
HF
H
+ + F
-Use the equilibrium equation to find out how much HF will form:
K
a=([H
+][F
-])/[HF]
Assuming you're adding a known amount of NaF you can calculate how much HF would form, and what the pH would be.
Hint: Use an I.C.E. (Initial, Change, Equilibrium) chart. Is there a significant amount formed at an initial pH of 7 (assuming you're using DI water)? To keep the math simple it's ok to use 1M as your initial NaF concentration since this is just a theoretical treatment of the problem.
Your reaction is correct except you didn't write it as an equilibrium. It's important to remember that this an equilibrium reaction.
The equilibrium equation for the reaction you wrote would be:
K
w/K
a=K
b=([HF][OH
-])/[F
-]
Note: pK
x=-log(K
x) therefore K=10
-pKx