Hi Shubhan,

We have a simple calculation to calculate the exact mass of one hydrogen atom

Mass of Proton = 1.008gram/mole

One mole of a gas at STP contains 6.023 * 10²³ atoms

Assuming negligible mass of electrons,

Mass of one atom of Hydrogen =  1.008g/mole * 6.023* 10²³ atoms/mole

= 1.67*10^(-30) g/atom

= 1.67*10^(-27) Kg/atom

Hope it help..!

Good Luck