The molar mass of a chemical substance is the mass of 6.022*10^23 elementary units. Hydrofluoric acid is made of molecules with chemical formula HF that have one atom of hydrogen and one atom of fluorine. The molar mass of hydrogen is 1 g/mole and the molar mass of fluorine is 19 g/mole. The molar mass of HF is 20 g/mole. Each set of 6.022*10^23 molecules of HF have a mass of 20 g.
The mass of 4.95*10^25 molecules is given by the expression (4.95*10^25/6.022*10^23)*20 which is equal to 1643 g
The mass of 4.95*10^25 HF molecules is 1.643 kg.