I tried this problem a lot of times, but just didnt get the answer. The molecular mass of helium is 4 g/mol, the Boltzmann’s constant is 1.38066 × 10−23 J/K, the universal gas constant is 8.31451 J/K · mol, and Avogadro’s number is 6.02214 × 1023 1/mol. Given: 1 atm = 101300 Pa. How many atoms of helium gas are required to fill a balloon to diameter 24 cm at 76◦C and 0.789 atm? What I did was find the volume and then use pv = nrt to find the moles. From the moles I found number of atoms. But it was wrong. Help Please.