[achibaby1974]

ok. totally lost on this lab.

we have to do:   2 Mg (s) + O2 (g)  2 MgO (s)

(a)  Mg (s) + 2 HCl (aq)  ----> MgCl2 (aq) + H2 (g); D H  = lab
(b)  MgO (s) + 2 HCl (aq) -----> MgCl2 (aq) + H2O (l); D H = lab

use these two equations and their enthalpy changes, along with this thermochemical equation:
(c)      H2 (g) + 1/2 O2 (g) H2O (l); D H = -285.4 kJ

Hess's Law of Heat Summation in order to predict the enthalpy change of the reaction:
(d)      Mg (s) + 1/2O2 (g) ----> MgO (s); D H = ?

ok. so, we did this and got our mass of magnesium, volume of HCl and T initial. We did it also for MgO mass, volume of HCl and T initial.
We measured the times then got our data. Then we had to plot the times into excel and find the final temp for all 6 plots by drawing the line. Ok. we got our final temps. We have 3 runs for Part 1 (Mg and HCl) and 3 Runs for Part 2 (MgO and HCl).

The first question is find Tfinal. Well, we have 6 of them and it only gives room for 3 runs not 6. Which final temp do we use? Mg or MgO? I think it's for Mg but I'm not sure.

Questions:

1.) T Final (FOR 3 runs but does not specify which one, Mg or MgO?)
2.) Delta T (for all 6 runs)
3.) Delta Heat of Formation (for all 6 runs)
4.) Give number of moles of product (all 6 runs)
5.) Molar Delta H not of reaction (Delta H/number of moles) (all 6 runs)   

1.) doesn't make sense b/c they don't specify which one. is it for Mg? Then you use the Final Temp of Mg to find the delta T for both Mg and MgO? This is a little confusing.

5.) Also, when it says to find the number of moles of product, which equation do we use, a, b, c or d? Which limiting reagents are we testing here?

[Rabn]

I'm going to give you the key to your 3 values problem in question 1 then leave 5 up to you.  This is a Hess' Law lab, therefore you must use Hess' law somewhere right? The key is to combine reactions a,b,and c to form the desired final reaction - 2 Mg (s) + O2 (g)  2 MgO (s) - What happens is you end up combining reactions a and b, the difference in delta T of the two is how you determine the answer to Q1.  I suddenly feel bad for telling you that....promise me you will really think about why you can do that.  In fact, you should tell me why you can do that.