How do you know which information given to you, is the one you use?

Question

For example, there was a problem that asks you how much heat was transferred, when 147g of NO2 is dissolved in 100g of water, and the equation was 3NO2 (g) + H2O (l) --> 2HNO2 (aq) +NO(g), and the delta H = -138kJ.

How do you know whether to calculate how much heat was released, by using 147g of NO2, or 100g of H2O?

Answer 1

You need to work out which is the limiting reagent. How many moles of NO2 and H20 do you have? Is there enough H2O to react all the NO2, or not?

NO2 has a molecular weight of 14.0 + 2*16.0 = 46.0 and H2O has a molecular weight of 18.0. So there are 147/46.0 = 3.20 mol of NO2 and 100/18.0 = 5.56 mol of H2O. Since we only need 1 mol of H2O for every 3 mol of NO2, it will take 3.20 / 3 = 1.07 mol of H2O to use up all the NO2. We have more than this amount of H2O, so NO2 is the limiting reagent.

So we have 3 mol of NO2 releasing 138 kJ of heat, from the equation; since we are reacting 3.20 mol we will have 3.20/3 * 138 = 147 kJ of heat released.

Answer 2

It helps if you label everything as completely as possible. For example, delta = -138kJ what-per-what-of-what.