Oh, well it says use density 1g/ml for solution so I just converted them into grams. And for the changing of signs, isnt the temperature already an indication if it's endothermic or exothermic? and I ONLY change signs when finding delta H because the Q absorbed by the solution, is the Q released by the reaction, therefore when finding deltaHrxn, I need to switch signs?
The density given is basically ignoring the weight of the HCl in the solution, if you had 50 mL of liquid HCl, and they told you to use density 1g/mL then you can just find the number of moles like you did before.
Temperature of the surroundings is an indicator of whether it is exothermic or endothermic, correct.
The 'proper' way you should have done your calculation is to use :delta:T as being Tfinal
which would have given you -0.81 o
C, then when you put this into your calculations you get a negative q.
The way q and H are related is that :delta:H = -q/n
So in the end two negatives gives you a positive and so the change in enthalpy is positive, meaning it is an endothermic reaction, but you could have deduced that from the change in temperature.
You have q, now you just gotta find 'n'