[hollytara]

Well acetic acid is only a little weaker - pKa 4.5

If you dissolve sodium acetate in water you get quite a bit of acetic acid. 

Its a standard general chem weak acid equilibrium problem:

Kb for F- is 1.5 x 10^-11  So F-  +   H2O     HF  +  OH-

start with 0.1 M F-  form x M of HF and OH-, end with 0.1 - x M of F-

1.5  x 10^-11  =  x^2 / (0.1 - x)   
x^2 = 1.5 x 10^-12 - 1.5 x 10^11 x
can be approximated as x^2 = 1.5 x 10^-12
x = 1.2 x 10^-6

so we would expect micromolar HF in a 0.1 M NaF solution; it is 3.8 x 10^-6 M  for 1 M NaF

LD50 for mice through the skin is 500 mg/kg

[Borek]

If you dissolve sodium acetate in water you get quite a bit of acetic acid.

About as handwavy as possible.

so we would expect micromolar HF in a 0.1 M NaF solution; it is 3.8 x 10^-6 M  for 1 M NaF

LD50 for mice through the skin is 500 mg/kg

Yes, and - translating these numbers into something that can be compared - the equilibrium concentration of HF in 1 M NaF solution is below 1mg/L range, several orders of magnitude too low to matter.

[rolnor]

Lets say you want to hurt someone in a pool by hydrofluoric acid, then NaHF2 will be more efficient then NaF but NaF will probably kill you as well by F- penetrating the skin.

[zorba9112]

Thanks everybody!
And thanks hollytara for the calculation.