Question

If 3.0 liters of oxygen gas react with excess carbon monoxide at STP, how many liters of carbon dioxide can be produced under the same conditions?

2 CO (g) + O2 (g) ---> 2 CO2 (g)

Answer 1

2CO + O2 ==> 2CO2
When using gases one may take a shortcut and use L directly as if they were mols.
3.0 L O2 x (2 mols CO2/1 mol O2) = 3 x 2/1 = 6 L CO2 produced.