theoretical yield

Elemental S reacts with O2 to form SO3 according to the reaction
2S+3O2 -> 2SO3

1) How many O2 molecules are needed to react with 6.18 g of S?
2)What is the theoretical yield of SO3 produced by the quantities described in Part 1?

1. convert to moles of sulfur

6.18g sulfur * ( 1 mol sulfur/32.065 g sulfur) = 0.1927 mol sulfur

2. find out how many moles of O2 needed
use conversion factors ( For two moles of sulfur, you need three moles of oxygen) and (1 mol equal to 6.022x10^23)
0.1927 mol sulfur x (3 mol O2/ 2 mol S) = 0.2890 mol of O2 x (6.022x10^ 23 molecules of O2/mol O2)
1.74*10^23 molecules of O2

3. Theo yield: use the either moles of O2 or moles of S to find the ratio. The easiest to use in this case is S because 2 moles of S produces 2 moles of SO3 so.....
0.1927 mol sulfur = 0.1927 moles of SO3

use conversion factor (80.06 g/mol for sulfur trioxide)

0.1927 mol sulfur x (80.06 g/mol of SO3) = 15.4 g SO3