This is a stoichiometry problem in which you calculate the amount of SO3 in units of liters at a certain temperature and pressure that are formed from the combustion of a certain amount of sulfur, which you will have to calculate. Overall, you will be following the "fabulous four steps" for stoichiometry problems:
Step 1: Write the balanced chemical equation for the reaction.
Step 2: Calculate the moles of "given" substance, (if more than one reactant amount is given, calculate the moles of each and determine which is the limiting reactant).
Step 3: Calculate the moles of "desired" substance from your answer in step 2 using the coefficients from the balanced chemical equation.
Step 4: Convert your answer in step 3 to the units the problem asks for. Usually this is grams, but it could be volume (for gases) or concentration (such as molarity, for solutions).
Step 1. The balanced chemical equation for the combustion of sulfur to sulfur trioxide is
S8 (s) + 12 O2 (g) 
8 SO3 (g)
Here I used the actual molecular formula for yellow sulfur which is S8, but we would get the same final answer if we just used "S (s)" instead, as some textbooks do.
Step 2 is to calculate the moles of "given" substance in the reaction, which is sulfur, S8. Take one percent of 1 kg, since that is the amount of sulfur in the one kg of coal, convert to grams, and then to moles using the molecular weight of S8.
Step 3 is where you use the coefficients in the balanced chemical equation to calculate the moles of "desired" substance, the SO3. It's just like a conversion problem starting with the moles of S8 you calculated in Step 2:
XXX mol S8 X 8 mol SO3 = YYY mol of SO3
1 1 mol S8
See how the moles of S8 cancel, leaving you with moles of SO3 in the answer. 
Step 4 will be to convert the moles of SO3 gas you calculated in Step 3 ("YYY") to liters, which you can do using the ideal gas law:
PV = nRT, V = nRT / P
And that will do it! 
Steve
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