This is, mostly, nonsense. Your question didn't speak to weight, it spoke of mass. It is generally agreed that elemental sulfur exists as an S8 ring. (cyclo-S8). (really we know it exists in a number of allotropes at STP) It is quite water insoluble, (with a solubility of 2½ ppm).
It is a basic assumption of chemistry (with the exception of radiochemistry) that mass is fixed (conserved) and does not change in any chemical process (including phase changes and dissolution). It is possible that when S dissolves in water, the S ring is broken up so that the molecular mass is decreased - I don't know. When it is burnt (oxidized with O2), the product's molecular mass is quite a bit less than the 256 of S8, meaning the "answer" is nonsense.
However. As any physicist knows E=mc². So an increase in an atom's ENERGY increases mass by E/c². Of course the energy change will be small in burning it, and c is a very large number, so that the tiny mass increase is not significant for a chemist.
Similarly, if S8 is dissolved in water cooler than it is, energy will be lost, but the same "insignificance" argument can be made. For a chemist (as I said, except for radioactive elements (or those in high energy environments like fission reactors, fusion experiments, or particle colliders (or when exposed to cosmic rays))) Mass is conserved, it does not change upon dissolution, burning, heating or cooling. The is no chemical element which changes mass upon dissolution or burning, although there are plenty of chemical compounds, including molecules of just one element, that may change molecular mass upon a phase change or dissolution, or that react to form products of more or less molecular mass.