[elfishies]

Consider a sample containing 2.00 mol of a monatomic ideal gas that undergoes the following changes from State A to State D:
State A PA=12.5atm, VA=14.00L
State B PB=12.5atm, VB=5.00L
State C PC=22.5atm, VC=5.00L
State D PD=22.5atm, VD=21.50L
Assume that the external pressure is constant during each step and equals the final pressure of the gas for that step.
Q: What is the value of q for the overall change from State A (through States B and C) to State D?


I'm lost and don't know where to start, can anyone tell me what formula to use? I know that q=deltaE+PdeltaV but i'm not sure if that's the right formula to use here, if it is how do I calculate deltaE? thanks!

[mjc123]

As all the steps are either at constant pressure or constant volume, I think the easiest way is
1. Calculate T for each state using the ideal gas law
2. For isovolumetric steps, q = 2*C_v,m*ΔT
3. For isobaric steps, q = 2*C_p,m*ΔT
Of course you know what C_v,m and C_p,m are for a monatomic ideal gas?

[elfishies]

I'm not sure if i'm doing this right

First i used PV=nRT --> T=PV/nR
state A: T=(12.5atm)(14L)/(2mol)(8.3145JK-1mol-1)=10.524K
state B: T=3.758
state C: T=6.765
state D: T=29.091

state A to B: q=n(5R/2)(delta T)--> q=2((5)(8.3145)/2)(3.758-10.524)=-281.249
B to C: q=2((3)(8.3145)/2)(6.765-3.758)=74.999
C to D: q=928.125

add them together and it equals 722 kJ?

[mjc123]

If you're using P in atm and V in L, what units does R need to be in?