Please help I am stuck on problem two and I think its a matter of calculating n,

Question

Please help I am stuck on problem two and I think its a matter of calculating n, which I seem to be getting wrong.

This is what my prof said but I am still getting the wrong answer some how.

"Ok, once again you're very close.  For part 2,

q = (Tb-Tc)*(3/2)Rn
you're forgetting n, the number of moles you have.  Without n, you're calculating the energy per mole.  You can find n from the information at point A, since you know the temperature (300k), the volume (0.5L), and the pressure (2 atm).
Another way of approaching this is remembering that PV = nRT, so nR(Tb-Tc) = (PbVb - PcVc)
q = (3/2)(PbVb - PcVc)"

A heat engine has the three step cycle shown above. Starting from point A, 1 L (liter) of monatomic ideal gas expands in the isobaric (constant pressure) process A to B at P = 2 atm. B to C is isovolumetric process at V = 5 L, and C to A is an isothermal (constant temperature) compression at 300 K.

1) Find the pressure (in atm) at point C in atmospheres atm.

Pc = _____________ atm

Pa*Va=Pc*Vc

2*1=Pc*5

Pc = 0.4 atm  = correct

2) Find the heat (in J) exhausted in the process B to C .

Q = __________________ J

q =(Tb-Tc)*(3/2)*Rn

I believe Tb is 1500K

Tc should v 300K

R=8.314

n=?  This is what is screwing me up I think.

3) If the net work done by the engine per cycle is 482.9 J, find the work done by the gas during process C to A.

WCA = ________________ J

Wab = 2atm(5-1) = 8 atm-liter = 810.6 J

Wnet = Wab - Wac

Wac = 810.6 - 482.9 = 327.7 J = correct

q = (Tb-Tc)*(3/2)Rn you're forgetting n, the number of moles you have. Without n, you're calculating the energy per mole. You can find n from the information at point A, since you know the temperature (300k), the volume (0.5L), and the pressure (2 atm). Another way of approaching this is remembering that PV = nRT, so nR(Tb-Tc) = (PbVb - PcVc) q = (3/2)(PbVb - PcVc)" A heat engine has the three step cycle shown above. Starting from point A, 1 L (liter) of monatomic ideal gas expands in the isobaric (constant pressure) process A to B at P = 2 atm. B to C is isovolumetric process at V = 5 L, and C to A is an isothermal (constant temperature) compression at 300 K. Find the pressure (in atm) at point C in atmospheres atm. Pa*Va=Pc*Vc 2*1=Pc*5 Pc = 0.4 atm = correct Find the heat (in J) exhausted in the process B to C . q =(Tb-Tc)*(3/2)*Rn I believe Tb is 1500K Tc should v 300K R=8.314 n=? This is what is screwing me up I think. If the net work done by the engine per cycle is 482.9 J, find the work done by the gas during process Wab = 2atm(5-1) = 8 atm-liter = 810.6 J Wnet = Wab - Wac Wac = 810.6 - 482.9 = 327.7 J = correct

Explanation / Answer

Applying ideal gas equation at point A,you will get n:

PV=nRT

=>n=PV/(RT)=2*1/(R*300)=1/(150R)

Now,

q =(Tb-Tc)*(3/2)*Rn

=>q=(1500-300)*(3/2)*R*(1/150R)=12J

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