A thermally isolated flask containing helium gas initially at 11 K, allows the g

Question

A thermally isolated flask containing helium gas initially at 11 K, allows the gas to slowly escape through a small tube until the pressure inside is 0.98 atm and the temperature is 4.2 K. For the flask to be entirely fillled with liquid at the end, what was the initial pressure?

Hint: Use the first law of thermodynamics equation and entropy. Assume the entropy of the remaining gas doesn't change. Find the entropy change on cooling and this will give you an equation. Also, use the entropy change on liquefying and this also will give you another equation. Combine the two equations with the help of other simple equations and you will find the initial pressure. This problem can be solved with five equations.

Explanation / Answer

from

gas law..

P1V1/T1 = P2V2/T2

V is constant

so P1 = P2T1/T2

P1= 0.98*11/4.2

P1 = 2.56 atm

P1 = 2.56 atm

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