Air of mass 1 kg. initially at 300 K and 10 bar, is allowed

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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/kg K, the change in entropy of air (in kJ/kg K, round off to two decimal places) is _________.

1. 0.66 kJ/kg K
2. 1.66 kJ/kg K
3. 0.33 kJ/kg K
4. - 1.66 kJ/kg K

Correct Option: A

Air, m = 1 kg

P₁ = 10 bar
R = 0.287 kJ/kg k Now,
∆s = S₂ – S₁

= mc_pln			T₂		0 - mRln			P₂
			T₁					P₁

= - 1 × 0.287 ln			1
			10

= 0.66 kJ/kg k
∴ ∆s = 0.66 kJ/kg k