What is the mass fraction of sulfur in sulfur oxide (6)?

Given:
Sulfur (VI) oxide – SO3

To find:
ω (S in SO3) -?

Decision:
1) Using the periodic system of D.I.Mendeleev, calculate the relative molecular weight of sulfur oxide (VI):
Mr (SO3) = Ar (S) * N (S) + Ar (O) * N (O) = 32 * 1 + 16 * 3 = 80;
2) Calculate the mass fraction of sulfur in sulfur oxide (VI):
ω (S in SO3) = N (S in SO3) * Ar (S) * 100% / Mr (SO3) = 1 * 32 * 100% / 80 = 40%.

Answer: The mass fraction of sulfur in sulfur (VI) oxide is 40%.