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In the reaction below, I know what A and B are, but I'm not sure what compound C is.

All I know about it is the following:

  • Deep yellow solid
  • Dication contains only sulfur
  • Raman and infra-red spectra of the dication show no common features

I think that the dication might be $\ce{[S4]^{2+}}$ since I think that this would be square planar and therefore have $D_{\mathrm{4h}}$ symmetry, which has no irreps that are both Raman and IR active, but I don't know what the anion would be.

The other likely options for the sulfur dication would be $\ce{[S8]^{2+}}$ or $\ce{[S19]^{2+}}$ but I think the fact that it's yellow would make it $\ce{[S4]^{2+}}$.

Reaction

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The reaction is given here.

$$\ce{S + 2H2SO4 → 3SO2 + 2H2O}$$

Sulfur react with sulfuric acid to produce sulfur dioxide and water. Sulfuric acid should be concentrated solution. The reaction takes place in a boiling solution.

$\ce{[S4]^2+}$, $\ce{[S8]^2+}$ or $\ce{[S19]^2+}$ is formed when sulfur is dissolved in oleum. The following is a paraphrase of an inorganic textbook.

enter image description here

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The yellow species is indeed the $\ce{S4^{2+}}$ cation, as reportrd by Nilay Ghosh's reference. Its relative stability and symmetric structure, amenable to delocalization of electrons, goes along with an inorganic version of aromaticity.

The counterion in the sulfuric acid medium would surely be leveled to bisulfate ion, $\ce{HSO4^-}$. The number of bisulfate ions is actually more than twice the number of $\ce{S4^{2+}}$ ions because the reduction process of concentrated sulfuric acid inherently generates more $\ce{HSO4^-}$ ions, and $\ce{H3O^+}$ ions, along with the acid reduction product. If we make the likely simplifying assumption that the acid which is reduced forms sulfur dioxide, we may render the reaction

$\ce{4S + 5H2SO4 -> S4^{2+} + SO2(g) + 4HSO4^- + 2H3O^+}$

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