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NEET levelSingle correct optionDifficulty 1 of 3td111

One mole of an ideal gas expands isothermally into an evacuated rigid container (free expansion). Which set of values is correct (IUPAC sign convention)?

A $q > 0, w < 0, Δ U = 0$
B $q = 0, w < 0, Δ U < 0$
C $q = 0, w = 0, Δ U = 0$
D $q < 0, w > 0, Δ U = 0$

Answer: C

The external pressure is zero, so

w = - P_{e x t} Δ V = 0

. The assembly is taken as thermally isolated, so

q = 0

. By the first law

Δ U = q + w = 0

, which is also expected because

U

of an ideal gas depends only on

T

and

T

is constant.

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