In which one of the following molecules does sulfur have the smallest oxidation number?

• A. SO2
• B. H2SO4
• C. H2S
• D. SO3

Answer: (C)

Verified step by step guidance

1. Recall that the oxidation number of sulfur depends on the oxidation states of the atoms it is bonded to and the overall charge of the molecule or ion.
2. Assign oxidation numbers to oxygen and hydrogen first, since their common oxidation states are well known: oxygen is usually -2, and hydrogen is usually +1.
3. For each molecule, write an equation expressing the sum of oxidation numbers equal to the overall charge (which is zero for these neutral molecules): for example, in SO2, let the oxidation number of sulfur be x, and oxygen is -2, so the equation is x + 2(-2) = 0.
4. Solve the equation for x (the oxidation number of sulfur) in each molecule: SO2, H2SO4, H2S, and SO3, by substituting the known oxidation numbers of oxygen and hydrogen and balancing the total charge.
5. Compare the calculated oxidation numbers of sulfur in all molecules to determine which one has the smallest (most negative or lowest) oxidation number.