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I got this reaction and I can't figure out the end products $$\ce{SO2 + S + CaO + Na2SO4 -> Na2S2O3 + ~?}$$ I am guessing some calcium sulfur salt should form, but which one?

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I am not one hundred percent certain, but I will try to give you some guidelines. There is the well known reaction of sulfur dioxide with calcium oxide. $$\ce{SO2 + CaO -> CaSO3}$$

Under aerobic conditions, this would further react with oxygen to form sulfate, since we have excess sulfur here I think it is fair to assume it will react to form thiosulfate. $$\ce{1/8S8 + CaSO3 -> CaS2O3}$$

Adding sodium sulfate is probably just a way to control/ buffer the pH during the reaction, I fail to see the actual relevance. But judging from the side reaction $$\ce{SO2 + 1/8S8 (s) + 3H2O <=> S2O3^{2−} + 2 H3+O}$$ it is actually necessary to have a buffer system based on sulfate. $$\ce{Na2SO4 + H3+O <=> 2Na+ + HSO4- + H2O}$$

I am not aware of any reaction of sulfur with sulfate.

In total I would write your equation like this: $$\begin{multline}\ce{SO2 (g) + S (s) + CaO (s) + Na2SO4 (s) ->[\ce{H2O}]\\ 2Na+ (aq) + Ca^{2+} (aq) + S2O3^{2-} (aq) + SO4^{2-} (aq)}\end{multline}$$

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  • $\begingroup$ Why is the meaning of $\ce{H3+O}$? Did you mean $\ce{H3O+}$? $\endgroup$ – Gaurang Tandon Jul 18 '18 at 3:08
  • $\begingroup$ @GaurangTandon in an attempt to display that the positive charge is not at the oxygen, I found it more suitable to write it this way. $\endgroup$ – Martin - マーチン Jul 18 '18 at 7:47
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First thing that came to my mind was calcium sulfate.

The unbalanced equation would be$$\ce{SO2 +S +CaO +NaSO4 ->Na2S2O3 + CaSO4}$$

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