Explanation:
There are 6.022x1023 molecules in 1 mole of SO3 (Avogadro's number) so in 0.25 moles, there are (0.25)(6.022x1023) molecules in the 0.25 moles of SO3. or 1.506x1023 molecules per 0.25 moles of SO3.
There is one atom of sulfur in each molecule of SO3. So there are 1.506x1023 atoms of sulfur in 0.25 moles of SO3.
