Calculate the dipole moment for hf (bond length 0.917 å), assuming that the bond is completely ionic.

The dipole moment  =  [the charge magnitude at one of the ends (you can choose either one)] [the distance between charges]
It is usually calculated in SI units of Debye which represents the positive and negative charges separated by 0.2082 angestron

Thus,
dipole moment = 0.917/0.2082 = 4.4044 debye.