The pKa of hydrogen fluoride, or HF, is 3.14. The pKa is a measure of the extent of acidity of an atom of hydrogen when it is part of a molecule. The value can be used to indicate the strength of an acid or a conjugate base.
Strong acids have pKa values of less than one, while the strength of conjugate bases increases with the increase of pKa values. The pKa should not be confused with the pH because, unlike pKa, pH measures the extent of acidity in a solution. Or, as Hyperphysics puts it, pH is a dependable measure of the extent of concentration of ions of hydrogen in a solution.