How many atoms are there in 3.4moles if helium ,show the calculation

20.4752789138x x  10^23 atoms
To count how many atoms in moles you need to know Avogadro's number. Avogadro's number dictate that for every mole there is 6.022140857 × 10^23 molecule/atoms.
Then 3.4 moles of helium will be 3.4x 6.022140857 x  10^23 atoms= 20.4752789138x x  10^23 atoms