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05/10/2020 5:55 pm
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A gas expands isothermally against a constant external pressure of 1 atm from a volume of 10 dm3 to a volume of 20 dm3. It absorbs 300 J of thermal energy from its surrounding. The ΔU is:
(a) – 312 J
(b) + 123 J
(c) – 213 J
(d) + 231 J
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05/10/2020 6:04 pm
(c) – 213 J
Explanation:
W = -∫V2 V1PdV = -P(V2 - V1)
W = –(1 atm) (20 – 10) = –10 dm3 atm.
= -10dm3 x 8.314 J/K/mol/ 0.08206dm3 /K/mol
= -1013J
According to Ist law of thermodynamics,
ΔU = q + W = 800J + (–1013 J) = –213 J