What is the oxidation number of sulfur in Na2S2O3?

(1) - 1 (3) + 6

(2) + 2 (4) + 4

Take it in pieces. First, you know Na has to be a + 1, and there are two of them. Hence, + 2 for that side. Now, oxygen. That is always - 2, and there are three of them. Therefore, that side is - 6. So what you have left unaccounted for to balance is + 4. BUT, there are TWO sulfur atoms, so each one of them must be + 2.

1. the answer is 3

2. the answer is 12