How many atoms are in 0.5 g Helium (He) ?

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