How do you do problems like...
3CO(g)+Fe2O3(s) ------> 2Fe+3CO2(g)+ 24.7KJ
How much heat is realsed when 56.0 of CO reacts to form the products.

Mmmmhh, you need to know 56 WHAT of CO are reacting (what's the unit of measure: grams, Kg, teaspoons, moles or what?) and 24.7KJ per which unit are given by your reaction (obviously a truckload of CO and iron oxide would give you a different amount of energy than a nanogram of it)