How to calculate (delta)H?

Since we are figuring out the heat of reaction based on the temperature increase of the water, we first need to know how much water we have. We added 100mL of HCL solution to 100mL of NH4 solution, so we have 200mL of water (.2L). Since 1mL of water is about 1 gram of water, we know we have 200 grams of water. We also know the heat capacity of water is 4.18J per gram per degree celcius, meaning it takes 4.18 joules to raise the temperature of one gram of water one degree C. Since our 200 grams of water was raised by a temperature of 2.34 degrees C, we just multiply 200g x 4.18 J/gC x 2.34 C to get our total number of J (see how the units cancel and we are left with just joules, or Delta H). However, we also must take into account the heat capacity of the calorimeter, whose temperature was also raised 2.34 degrees. It takes 480J to raise the calorimeter one degree, so multiple 480J x 2.34 to get the joules of heat needed to raise the calorimeter temperature. Now add the two values (joules required to raise the water's temperature and joules to raise the calorimeters temperature) and that is your overall delta H for this reaction. However, it's asking for the delta H per MOLE of reaction. Since we only have .2 moles of each substance (and thus .2 moles of reaction overall), we must multiple our answer by 5 to get the delta H for one mole of reaction. So just add up the joules and multiply by 5 and you're good. I would ask your professor what to do, though, when it's a reaction where it's something like 2A + 3B = 2C, since that is technically one mole of reaction but we can obviously see that we have 2 moles of A and 3 moles of B to equal one mole of reaction, and that takes more time to explain.