Kinetic Theory of Gases

We know $v_{rms} = \sqrt{\dfrac{3RT}{M}}$

$\implies 300 = \sqrt{\dfrac{3RT}{4 \times 10^{-3}}}$

$\implies 300^2 = \dfrac{3RT}{4 \times 10^{-3}}$

$\implies RT = 120$ .

Now For each direction, that is x,y,z axis, Energy is $\dfrac{nRT}{2}$

So By Law Of Equipartition of energy,

$E_{k} = \dfrac{3nRT}{2}$

So Putting value of $n=8/4 = 2$ and $RT=12$

$E_{k}=360 kJ$ [ANS]