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HeatQ=(3/2)RT is supplied to 4 moles of an ideal diatomic gas at temperature T. How many moles the gas are dissociated into atoms if the temperature of gas is constant?
(A) 3
(B) 2
(C) 1
(D) Data insufficient

Answer
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Hint Find the change in the internal energy for initial 4 moles of diatomic gas and final dissociations into atoms. One mole of diatomic gas becomes two when it gets dissociated into monoatomic gas. The internal energy for a particular type of gas depends upon how many degrees of freedom the gas has. Monoatomic gas has three degrees of freedom and diatomic gas has five degrees of freedom.
FORMULA USED: Q=ΔU=UfUi,
U=52NKT for diatomic gas
 U=32NKT for monoatomic gas
Where Uf → Final Internal Energy
N → No. of Moles
Ui → Initial Internal Energy
K → Boltzmann Constant
T → Temperature

Complete Step By Step solution
GivenQ=(3/2)RT, Moles of diatomic gas = 4
Let the moles dissociated be ‘x
1 mole of diatomic gas becomes 2 moles of monatomic gas when the gas is dissociated into atoms.
So, if xmoles of diatomic gas are dissociated then they will form 2xmoles of monatomic gas.
So, Uf (final internal energy) =(2x×32RT)+((4x)×52RT) [R=NK]
R→ ideal gas constant for monoatomic gas for diatomic gas
And Ui (initial internal energy) =4×52RT (as all moles are of diatomic gas before dissociation)
So, Q=UfUi
32RT=(2x×32RT+(4x)×52RT)4×52RT
32RT=3xRT+10RT5x2RT10RT
32RT=3xRT5x2RT
Dividing both sides byRTwe get, 32=3x5x2
32=6x5x2
32=x2
So,
x=3
So, 3 moles of the gas are dissociated into atoms if temperature is kept constant.

So, option (i) is correct.

Additional Information Kinetic Energy is the only one, contributing to the internal energy. Each degree of freedom contributes, 1/2KT per atom to the internal energy. For monatomic ideal gases with Natoms, its total internal energy U is given as, U=32NKT and for diatomic gases, U=52NKT
The degrees of freedom (F) for monatomic gas is 3 and for diatomic gas is 5.
Degree of Freedom: It is the no. of values in the final calculation of a statistic that are free to vary or we can say that it is the no. of independent ways by which a dynamic system can move, without violating any constraint imposed on it.
Monoatomic gas has three translational degrees of freedom. These are due to the motion of the centre of mass of that molecule alongxaxis, yaxisand zaxis
Diatomic molecule can rotate about any axis at right angles to its own axis. Hence it has two degrees of freedom of rotational motion in addition to three degrees of freedom of translational motion.

Note Concepts of thermodynamics and its laws should be studied well. Degrees of Freedom of various types of molecules should be known along with their internal energies. The role of temperature can be studied and its effect on the variation of internal energy.