Well first you need to know the molar mass of He which is 4.0026g/mol and Avogadro’s number which is 6.022*10^23 (units). So the equation is
0.5gHe * mol/4.0026He *(6.022*10^23)He atoms/mol
=7.5*10^22 He atoms
Round to only one sig fig because 0.5g only has one
Final answer: 8*10^22 He atoms
