How many moles of hf acid are there in an 80g sample?

             3.98 mol

Solution:
              Moles and mass is related to each other as follow,

                                Moles  =  Mass / M.mass  (1)

Data Given:
                  Mole  =  ?

                  Mass  =  80 g

                  M.mass  =  20.07 g/mol    ∴ (H = 1.08 g/mol) + (F = 18.99 g/mol) 

Putting values in equation 1,

                Mole  =  80 g ÷ 20.07 g.mol⁻¹

                Mole  =  3.98 mol