Two thiosulfate ions are oxidised to one tetrathionate ion: $2 S X_{2} O X_{3} X^{2 -} S X_{4} O X_{6} X^{2 -} + 2 e X^{-}$ . Using the AVERAGE oxidation number of S in each ion (O is $- 2$ ), by how much does the total oxidation number of all sulfur atoms increase for this change?

Answer: A

Thiosulfate:

2 x - 6 = - 2

gives

+ 2

, so four S total

+ 8

. Tetrathionate:

4 x - 12 = - 2

gives

+ 2.5

, so four S total

+ 10

. The increase is

10 - 8 = 2

, matching two electrons lost.