Hello; i'm having some difficulties with a thermo question of mine.(adsbygoogle = window.adsbygoogle || []).push({});

at low density water vapor conforms well to the ideal gas equation provided the temp is higher than about 320K, but the heat capacity is a function of temperature. The following formula gives the specific heat capacity at constant volume, as a function of T

cv = 1273.0 + 0.3441T + (2.833x10^-4)T^2 J/kgK

a) calculate the entropy change for 1kg of water vapour heated from 350K to 1000K at constant volume.

Here's what i did; S = (integral)dQ/T = (integral)NcvdT/T. Where N = (m/MM) = 1/0.018

I subbed cv into this equation then divided it by T

= (m/MM)(1273.0/T + 0.3441 + (2.833x10^-4)T)dT

Then integrated over T1 = 350, T2 = 1000 to get:

S = (1/0.018)(1273.0(ln(1000/350)) + 0.3441(1000-350) + ((2.833x10^-4)/2)(1000-350)^2)

however, my answer is not correct according to the back of the book. The answer is supposed to be 1.68kJ/K. Where did i go wrong?

b) Same as a) except at constant pressure.

here's what i tried;

S = (inegral)(Ncpdt)

where cp = cv + R

hwoever this also doesn't give me the correct answer.

Any help or suggestions would be greatly appreciated!

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# Homework Help: Thermo Help!

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