chemistry

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My chem book gives me the following Hess' Law example:

Mg + 2HCl --> H2 MgCl2 Delta H1

MgO +2HCl --> H2O +MgCl2 Delta H2

H2 + 1/2 O2 --> H2O Delta H3=-68.3kcal
The desired reaction is the formation reaction of MgO as follows:
MgS +1/2 O2 --> MgO Delta H final= function of all the delta H's above.
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I can pretty much understand Hess' Law pretty well, it's just the after quiz questions that have me a little off, such as:

There is a correction factor of about 0.3 kcal/mole for the magnesium reaction because of the evolution of hydrogen gas. Explain the cause of this correction, and should it be added or subtracted from Delta H1?

(My guess would be added, but not completely sure)

The heat capacities of your containers and probe used if you were preforming this experiment, would be ignored in your calculations, causing a small error. What would be one way the heat captivity could be measured experimentally?

(No idea)

And does the error mentioned above cause the calculated heats of the reactions to be less negative or more negative than they should be?
(More negative?)

It is best if MgO is heated to 400 degrees-500 degree Celsius for a few hours before you use it. Why?
(No idea)

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