The standard molar volume is 22.4 L, so 12.61 L is 12.61 / 22.4 = 0.5629 moles of SO2 .
The molar ratio of S on the reactant side to S on the product side is 1 to 1.
(In other words, one mole of solid S atoms yields one mole of SO2 molecules )
Thus 0.5629 moles of solid S must be used. Since the molar mass of S is 32.065 (from periodic table),
the mass of sulfur needed is 0.5629 x 32.065 = 18.05 grams.
