Formula for Sodium Thiosulfate: Na2S2O3 - Jules

[Jules18]

How would I figure out the formula for Sodium Thiosulfate?

I was asked that question on an exam and I knew sodium sulfate was Na₂SO4, so I figured ok, thio means sulfur, so I'll just add another S like this:

Na₂S₂O₄

But my prof said Na₂S₂O₃
was the right answer, and I have no idea how I was supposed to know there were only 3 O's, since it's not sulfite.
Wikipedia agrees with him, tho.

Any ideas?

~Jules~

 

[KevinMcGovern]

You're replacing an Oxygen atom with a Sulfur atom. The original compound is Sodium Sulfate, which is Na₂SO₄, but when it becomes Sodium Thiosulfate, one of the Oxygen atoms is replaced with a Sulfur atom. So it becomes N₂S₂O₃.

If it were to be Sodium Dithiosulfate, you would replace 2 Oxygen atoms with 2 sulfur atoms, so it would be N₂S₃O₂

 

[lightarrow]

How would I figure out the formula for Sodium Thiosulfate?

I was asked that question on an exam and I knew sodium sulfate was Na₂SO4, so I figured ok, thio means sulfur, so I'll just add another S like this:

Na₂S₂O₄

But my prof said Na₂S₂O₃
was the right answer, and I have no idea how I was supposed to know there were only 3 O's, since it's not sulfite.
Wikipedia agrees with him, tho.

Also, consider that in your formula: Na₂S₂O₄ sulfur would have oxidation number = 3, which doesn't exist (for sulfur).