Mr(SO3) = Ar(S) + 3*Ar(O) = 32 + 3*16 = 80

ω(S) = (Ar(S)/Mr(SO3))*100% = (32/80)*100% = 40%

ω(O) = 100% - ω(S) = 100% - 40% = 60%

Ответ: ω(S) = 40%, ω(O) = 60%