Question

When a mixture of 10 moles of SO2 and 15 moles of O2 was passed over a catalyst, 10 moles of SO3 was formed. how many moles of SO2 and O2 did not enter into combination?

Answer 1

2SO2 + O2 ==> 2SO3

The long way of solving but easiest to explain is this.
Convert 10 mols SO2 to mols SO3.
10 x (2 mols SO3/2 mols SO2) = 10

Convert 15 mols O2 to mols SO3.
15 x (2 mols SO3/1 mol O2) = 30 mols SO3.

So we know 10 mols SO3 is the limiting reagent. How much O2 will that use?
10 mols SO3 x (1 mol O2/2 moles SO3) = 5 mols O2. So we use all of the SO3(leaving zero not reacted), we use 5 of the 15 (leaving 10) moles O2 not reacted.

Answer 2

just use formula idiot

Answer 3

So2=2 .O2=11