A gas expands isothermally against a constant external pressure of 1 atm from a volume of 10 `dm^(3)` to a volume of 20 `dm^(3)`. It absorbs 800 J of thermal energy from its surroundings. The `Delta`U is

Options

A. ` -312 J`
B. + 123 J
C. `-213 J`
D. +231 J

Answer

C

Explanation

From 1st law of thermodynamics
`DeltaU= q+W`
`Q= +800 J`
`W=- P(V_(2)-V_(1))= -1(20-10)= -10 dm^(3)atm= - (10dm^(3) xx 8.314 J//K mol)/(0.08206 dm^(3) // Kmol)`
W= -1013 J
`Delta U= 800 J+(-1013J)= -213 J`