stoichomtryof gases

Question

5. What mass of sulfur must be used to produce 12.61 L of gaseous sulfur dioxide at STP according to the following equation?
S8(s) + 8O2(g) ? 8SO2(g)

Answer 1

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.