 Okay, so this one is the number 10 on the practice exam. Consider the following reaction. How much energy is released when 2.000 moles of H2O is formed? Okay, so if we look at this reaction, we've got a 1 to 11 to 7 to 8 ratio here. Okay, 2 for every negative 4,130 kilojoules. So for every 8 moles of water you produce, you produce also negative 4,130 kilojoules. So every 8 moles of this you produce, you produce that much heat. Remember with an exothermic reaction, heat or delta H is like a product. So if you're thinking, well how much energy is released when I have 2 moles of water being formed? So it's going to be, what is the delta H if we have 2 moles H2O? Okay, you know that for every 8 moles of H2O you're going to have negative 4,130 kilojoules. So 4,130 kilojoules divided by 8 moles H2O. But we're concerned only about 2 moles of H2O. So you're just going to multiply that 2,000 H2O. Of course now moles of H2O cancel out there, cancel out there. So now you just take 1130, divide that by 8 and then multiply it by 2. And you get negative 1,032.5 kilojoules. Of course when you do this you see, well there's no decimal point there. So I guess this would be 3 sig figs. So you're going to have to take this to 3 sig figs. So what I would do is negative 1.03 times 10 to the 1,2,3. Does that make sense though? So you always want to remember that the reaction equation gives you most of the information you're going to need. You just got to convert it from moles to grams or whatever you need.