To find the number of molecules, we'll use the formula:

Number of moles = \(\frac{Number of molecules}{Avogadro number}\)

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 = \(\frac{mass}{molar mass}\) = \(\frac{11.5}{4}\) = 2.875 moles

Number of molecules = Number of moles x Avogadro number

 = 2.875 x 6.02 x 10\(^{23}\)

 = 17.3075 x 10\(^{23}\)

 = 1.73 x 10\(^{24}\)