The molar mass of SO3 = 80.066 g The mass of sample SO3 = 78.0 g Let, the no. of molecules of SO3 be \\(n\\) No. of molecules in a sample of a compound of mass \\(m\\) and molar mass \\(A\\) is, \\(\\mbox{No. of molecules($n$)} = \\frac{\\mbox{mass of the sample($m$)}}{\\mbox{molar mass of the compound(A)}} \ imes (6.022 \ imes 10^{23})\\) So, using this formula we calculate the number of molecules of SO3. Hence, \\(n = \\frac{78.0}{80.066} \ imes 6.022 \ imes 10^{23}\\) \\(n = 5.458 \ imes 10^{23}\\) So, the number of molecules in 78 grams is \\(5.458 \ imes 10^{23}\\).