Comparing Carnot efficiency to Stirling efficiency

[WalkTex]

Homework Statement

In the problem, we were to derive work and heat for each leg of the cycle. Then we are asked:

In the definition of the efficiency of this cycle, η =W Qpos , where Qpos is the total positive heat flow to the engine, what is the efficiency of the cycle when Th = 700K and
Tc = 400K? V1 = 0.5 L and V2 = 1.5 L. Compare this efficiency to the efficiency of a Carnot cycle that operates between the same temperature extremes.

Homework Equations

Work = integral from Vi to Vf of PdV
W done during cycle = nr(Th-Tc)ln(vf/vi) Derivation here:

http://www.pha.jhu.edu/~broholm/l39/node5.html

Q = Cv(Th -Tc) - nRT ln (Vf/Vi)

We are told Cv = 5/2 * R

efficiency of a carnot cycle = 1- Tc/Th

The Attempt at a Solution

I have attempted to get the solution but have come up with a result that is about an order of magnitude too small that i am unable to rectify. It is part c in the attachment.

 

[TSny]

C_V = 5R/2 is the molar heat capacity at constant volume for a diatomic gas. You need to take account the number of moles when using C_V. (You didn't specify if the number of moles is given or whether the gas is monatomic or diatomic.)