Specific Heat Capacity vs Cv(Vap) vs Cp(vap) [ENDORSED]

Michael_Johanis_2K
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Specific Heat Capacity vs Cv(Vap) vs Cp(vap)

"A pressure cooker containing 100.0 g of liquid water at 25 C is placed on the stove until all water becomes superheated steam at 121 C. Assume no steam escapes the pressure cooker.

a) Find q from 25C to 100C
b) Find q during boiling
c) Find any other heat gained or lot by the water as it goes from 25C to 121C

For Part C, I used q = m C DeltaT.
m = 100.0 g, T = 21C, C = 2.01 J/(Cg), which is the specific heat capacity of water vapor. Why is my value of C wrong? I was supposed to use Cv(vap) but I have never used this before on questions involving fusion/vaporization. I have always used the specific heat capacity of (material) vapor -- water vapor in this case, which was wrong. Can someone please explain?

Chem_Mod
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Re: Specific Heat Capacity vs Cv(Vap) vs Cp(vap)  [ENDORSED]

A pressure cooker is constant volume as opposed to constant pressure. The standard heat capacity you've been using is the constant pressure heat capacity $C_p$ which is applicable for cases in which the reaction takes place under a constant atmospheric pressure. Because the pressure cooker is constant volume and not constant pressure (the pressure is actually increasing in the cooker), the constant volume specific heat must be used.