For an elastic or inelastic collision, momentum is always preserved.
The molecule approaches and rebounds with the same speed of $200 \mathrm{~m} / \mathrm{s}$.
$u=v=200 \mathrm{~m} \mathrm{~s}^{-1}$
Therefore, Initial kinetic energy will be,
$(1 / 2) \mathrm{mu}^{2}=(1 / 2) \mathrm{m}(200)^{2}$
Final kinetic energy will be,
$=(1 / 2) m v^{2}=(1 / 2) m(200)^{2}$
Therefore, kinetic energy is also conserved.