[sally125]

I thought I was getting good at these questions but I am quite stumped by this one.  Using Hess' law I found that the deltaH of Mg(s) + 2 H2O(l) -> Mg(OH)2(s) + H2(g) is -638.71kJ but I don't know where to go from there.  Any guidance would be greatly appreciated.

The U.S. military uses a “flameless heater” to warm pre-prepared meals for active service
personnel. The heater contains magnesium metal, which reacts with added water to form
magnesium hydroxide in a very exothermic, oxidation-reduction reaction:
Mg(s) + 2 H2O(l) -> Mg(OH)2(s) + H2(g)
The reaction generates enough heat to cook a meal enclosed in a plastic and aluminum foil pouch.
Given the following data:
Hof(H2O,l) = –285.83 kJ mol–1   (Hof being the formation)
Hof(Mg(OH)2,s) = –924.54 kJ mol–1
specific heat of water = 4.184 J g–1 °C–1 density of water = 1.00 g mL–1
how much magnesium metal is needed to supply the heat required to warm 25.0 mL of water from
25°C to 85°C?

[Donaldson Tan]

how much heat does 1 mole of Magnesium produce?

how much heat is needed to raise the temperature of 25ml of water by 40^oC?

how much magnesium is need to produce the the amount of heat needed to raise the temperature of 25ml of water by 40^oC?

[sally125]

Your questions were very helpful and I had done Hess' law wrong.  Thank you.