2kg of a monoatomic gas is at a pressure of 4 × 10 ^ 4 N m ^ - 2 ....

Internal energy is the energy due to thermal motion.

The internal energy of an ideal gas given by

$U = \dfrac{f}{2}nRT$ (for $nmoles$)

$U = \dfrac{f}{2}\left(PV\right) = \dfrac{f}{2} \times P \times \dfrac{M}{ρ}$ (Since, $f = 3$ for a monoatomic gas.)

$U = \dfrac{3}{2} \times 4 \times {10}^{4} \times \dfrac{2}{8}$

$= 1.5 \times {10}^{4}J$
So, the order of energy due to thermal motion is ${10}^{4}$