Step 1
The reaction involved:
2 S + 3 O2 => 2 SO3
-----------
Step 2
Data provided:
The number of moles:
S) 3.0 moles
O2) 4.0 moles
------------
Step 3
Procedure:
By stoichiometry,
2 S + 3 O2 => 2 SO3
2 moles S ------ 3 moles O2
3.0 moles S ------- X
X = 3.0 moles S x 3 moles O2/2 moles S
X = 4.5 moles
For 3.0 moles of S, 4.5 moles of O2 are needed and there are just 4.0 moles of O2, so the limiting reactant is O2.
(Remember that the limiting reactant limits the formation of product and it is consumed first)
Answer: The limiting reactant is O2
