Let u be velocity of hydrogen atom before collision and V the ve locity of two atoms moving together after collision By principal of conservation of momentum , we have
Mu+M×0=2MV
V=u2
The loss in kinetic energy Δϵ due to collision is
Δϵ=12Mu2−12(2M)V2
As V=u2, we have Δϵ=12Mu2−12(2M)(u2)2
=12Mu2−14Mu2=14Mu2
This loss in energy is due to excitation of one of hydrogen atoms . The ground state (n=1) energy of a hydrogen atom is ϵ1=−13.6eV
The energy of first excited level (n=2) is :ϵ2=−3.4eV
Thus minimum energy required to excite a hydrogen atom from ground state to first excited state is :
ϵ2−ϵ1=[−3.4−(13.6)]eV=10.2eV
=10.2×1.6×10−19J=16.32J
As per problem with the loss in kinetic energy in collision is due to energy used up in exciting one of atoms
Thus, Δϵ=ϵ2−ϵ1
or 14Mu2=16.32×10−19
or u2=4×16.32×10−19M
The mass of hydrogen atom is 1.0078amu or 1.0078×1.66×10−27kg
u2=4×16.32×10−191.0078×1.66×10−27=39.02×108kg
or u=√39.02×108=6.24×104ms−1