The pKb for fluoride ion at 25°C is 10.83, the ionisation constant of hydrofluoric acid at this temperature is

F-+H2O⇌HF+OH-Kb=[HF]OH-F-Kw=H3O+OH-=10-14Dissociation of HF in water is represented by the equation,HF+H2O⇌H3O++F-Ka= H3O+F-[HF]-----------(iii)Kb·Ka=H3O+OH-=Kwor KwKb=KaTaking log on both sideslogKa= logKw-logKb          =-pKw+logKb          =-14+10.83=-3.17     Ka= 6.76×10-4