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Physics Question

A 10-g sample of a cookie is allowed to dry before putting it in a bomb calorimeter. The aluminum bomb has a mass of 0.615 kg and is placed in 2.00 kg of water contained in an aluminum calorimeter cup of mass 0.524 kg . The initial temperature of the system is 15.0°C , and its temperature after ignition is 36.0°C . The value of specific heat for aluminum is 0.22 kcal/kg⋅C° , and for water is 1.00 kcal/kg⋅C° .
 
(a) Determine the energy content of 100g of cookies in kcal.
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1 Answer

Meg,
The answer to your other question should set you on track to answer this one.

Comments

Q (for 10g) = Sigma(all changes) = ((0.615 kg+0.524 kg)*21C*0.22 kcal/kgC)+ (2.00 kg * 21C * 1.00 kcal/kgC)
Scale up by factor of (100g/10g) and you should be there. I get ~473 kcal. (did you forget the scaleup?)
Note: Combustion is defined as producing products at standard conditions (i.e. liquid water and gaseous CO2): should you worry about that? Well, heats of combustion should be ΔH, not ΔU (the latter also known as ΔΕ) -- i.e. at constant pressure -- but, depending on your course, you may not need to compensate for the ΔPV work done on the system when you "relax" the bomb products to constant pressure (it's slightly positive, since a little O2 was consumed to make H2O, so that the surroundings actually adds a little work to the system when you replace that volume lost: that will reduce slightly the net energy produced by the "system", hence reduce that 473 value a little -- I estimate by about 1 kcal [reasoning: CH2O makes H2O; 30g carbohydrate uses 1 mol O = 1/2 mol O2 ; then 100g carbohydrate uses 10/6 mol O2; ΔΡV = nRT, you do the rest]; note that the O2 to CO2 transition is a "wash" -- there's no moles of gas change.)
I'll assume that if your course has covered stoichiometry of reactions, you can calculate the ΔPV correction from the formula for carbohydrate, if you need it, I've just given you a skeletal outline above.
I did scaleup, but I scaled up by 100 instead of 10 for some reason. Thank you so much!