To find the number of molecules, we'll use the formula:
Number of moles = Number of moleculesAvogadro number\;\frac{\text{Number of molecules}}{\text{Avogadro number}}Avogadro numberNumber of molecules
But We were only given the mass of Helium = 11.5g , knowing the atomic mass or molar mass of He = 4 g/mol
Number of moles = massmolar mass\;\frac{\text{mass}}{\text{molar mass}}molar massmass = 11.54\;\frac{11.5}{4}411.5 = 2.875 moles
Number of molecules = Number of moles x Avogadro number
= 2.875 x 6.02 x 1023^{23}23
= 17.3075 x 1023^{23}23
= 1.73 x 1024^{24}24
