Step-by-step explanation:
To calculate the number of molecules, we need to first find the number of moles of SO3 using its molar mass.
Molar mass of SO3 = 32.06 g/mol (sulfur: 32.06 g/mol, oxygen: 3 x 16.00 g/mol)
Number of moles = Mass / Molar mass
= 280.4 g / 32.06 g/mol
= 8.74 mol
Now, we can use Avogadro's number to calculate the number of molecules.
Avogadro's number = 6.022 x 10^23 molecules/mol
Number of molecules = Number of moles x Avogadro's number
= 8.74 mol x 6.022 x 10^23 molecules/mol
= 5.26 x 10^24 molecules
Therefore, a sample of 280.4 g of SO3 would contain approximately 5.26 x 10^24 molecules.