JEE MainPhysicsHeat and ThermodynamicsMCQ+4 / −1
One mole of an ideal gas is taken through an adiabatic process where the temperature rises from 27 C to 37 C. If the ideal gas is composed of polyatomic molecule that has 4 vibrational modes, which of the following is true? [R = 8.314 J mol 1 k 1]
- Awork done by the gas is close to 332 J
- Bwork done on the gas is close to 582 J
- Cwork done by the gas is close to 582 J
- Dwork done on the gas is close to 332 J
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Correct answer: B
- Use the first law for an adiabatic process
For an adiabatic process, So from the first law, where is the work done by the gas.
Thus,
- if , then
- meaning work is done on the gas.
Here temperature increases from to so Hence internal energy increases.
- Find degrees of freedom of the gas
The gas is polyatomic and has 4 vibrational modes.
For a non-linear polyatomic molecule:
- translational degrees of freedom
- rotational degrees of freedom
- each vibrational mode contributes quadratic degrees of freedom
So total degrees of freedom:
Therefore,
- Calculate change in internal energy
For 1 mole ideal gas, With , Using , So,
- Relate this to work
Since adiabatic, This means the gas does negative work, i.e. 582 J of work is done on the gas.
- Check options
- A: work done by the gas is close to 332 J ❌
- B: work done on the gas is close to 582 J ✅
- C: work done by the gas is close to 582 J ❌
- D: work done on the gas is close to 332 J ❌
So the correct option is
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