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Question

One mole of a liquid (1 bar, 100 ml) is taken in an adiabatic container and the pressure increases steeply to 100 bar. Then at constant pressure of 100 bar, volume decrases by 1 mL. Calculate ΔH.

A
500 J
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B
990 J
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C
650 J
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D
720 J
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Solution

The correct option is B 990 J
From first law of thermodynamics,
ΔU=q+w
But as the process is carried out under adiabatic condition, q=0.
Hence ΔU=w
Further, at constant pressure of 100 bar, volume has decreased by 1 mL, therefore, work of contraction will be :
w=PΔV=100 bar×1mL
=(100×105Nm2)(106m3)=10J

Hence, ΔU=10J
Enthapy change is given as:
ΔH=ΔU+Δ(PV)
Δ(PV)=(P2V2P1V1)

P1=1 bar, V1=100 mL
P2=100 bar, V2=99 mL

Δ(PV)=(100×99)(1×100)
Δ(PV)=9800 bar mL=980J
as 1 L bar=101.3 J
so,
ΔH=10J+980J=990J

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