Question

How many moles of oxygen can be obtained by the decomposition of 7.5 mol of reactant in the following reaction?
2KClO3 -> 2KCl + 3O2

Answer 1

Here is the exact problem I've worked out. Start with moles (you can omit the equation step and converting grams to moles step).
http://www.jiskha.com/science/chemistry/stoichiometry.html

Answer 2

That would be 11.25 mol O2

Process:
7.5 mol KClO3 x 3 mol O2/2 mol KCOl3 = 11.25 mol O2
Note: Cancel the mol KCLO3