Homework Help: Thermodynamics problem help again

1. Dec 12, 2004

Enoch

Sorry for the second question in just a day, but my proffessor really didn't cover this material very well. Thanks for any help given :).

A cylinder contains 4.22 mol of helium gas at a temperature of 314 K. The molar specific heat of helium at constant volume is 12.5 J/K mol. How much heat must be transferred to the gas to increase its temperature to 531 K if it is heated at a constant volume? Answer in units of J.

Well, after reading the problem, I knew that it was an Isochoric Thermal process due to the comment "heated at a constant volume". Therefore this equation applies:

Change in Internal Energy = Q - W
- The work is going to be 0 joules
- Amended equation: Change in internal Energy = Q

Q = Cn(Tf - Ti)
- Solving for that: Q = (12.5)(531 - 314) = 2712.5

I plugged that in as my answer, and it was wrong. Where did I go wrong? Is the problem even setup right from my perspective?

2. Dec 12, 2004

dextercioby

Brilliantly,up until the final calculation.U forgot to multiply by the number of moles of He.Your result should ve been 4.22 times larger.It's awkward,because u stated the formula right.

3. Dec 12, 2004

Enoch

Again, thanks very much.