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Enthalpy

Air of mass 1...

Question

Air of mass $1$ kg, initially at $300$ K and $10$ bar, is allowed to expand isothermally till it reaches a pressure of $1$ bar. Assuming air as an ideal gas with gas constant of $0.287$ kJ/kgK, the change in entropy of air (in kJ/kgK, round off to two decimal places) is
0.66

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Solution

The correct option is A 0.66
$m = 1 k g; P_{1} = 10 b a r = 1000 k P a; T_{1} = 300 K$
$T_{2} = 300 K, P_{2} = 1 b a r$
$S_{2} - S_{1} = m c + p l n \frac{T_{2}}{T_{1}} - m R l n \frac{P_{2}}{P_{!}}$
$= - m R l n \frac{P_{2}}{P_{1}} = - 1 \times 0.287 l n \frac{1}{10}$
$= 0.66 k J / k g K$

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