Let the insulated container has n moles of the monoatomic gas. The loss of kinetic energy of the gas, ΔEK​=21​(mv)v2 If the change in temperature of the gas is ΔT, then heat gained by the gas is, ΔQ=23​nRΔT Now, according to question, ΔQ=ΔEK​23​nRΔT=21​mnv2⇒ΔT=3Rmv2​