The formula for a sulfur compound, in which the mass fraction of sulfur is 50%: 1) SO3 2) H2SO3 3) SO2 4) H2SO4

Mr (SO3) = Ar (S) * N (S) + Ar (O) * N (O) = 32 * 1 + 16 * 3 = 82;
ω (S in SO3) = N (S) * Ar (S) * 100% / Mr (SO3) = 1 * 32 * 100% / 80 = 40%;
Mr (H2SO3) = Ar (H) * N (H) + Ar (S) * N (S) + Ar (O) * N (O) = 1 * 2 + 32 * 1 + 16 * 3 = 82;
ω (S in H2SO3) = N (S) * Ar (S) * 100% / Mr (H2SO3) = 1 * 32 * 100% / 82 = 39.02%;
Mr (SO2) = Ar (S) * N (S) + Ar (O) * N (O) = 32 * 1 + 16 * 2 = 64;
ω (S in SO2) = N (S) * Ar (S) * 100% / Mr (SO2) = 1 * 32 * 100% / 64 = 50%;
Mr (H2SO4) = Ar (H) * N (H) + Ar (S) * N (S) + Ar (O) * N (O) = 1 * 2 + 32 * 1 + 16 * 4 = 98;
ω (S in H2SO4) = N (S) * Ar (S) * 100% / Mr (H2SO4) = 1 * 32 * 100% / 98 = 32.65%.

Answer: option 3).