Two perfect gases at absolute temperatures T1 and T2 are mixed. There is no loss of energy in this process. If n1 and n2 are the respective number of molecules of the gases, the temperature of the mixture will be
n1T1+n2T2n1+n2
Average kinetic energy per molecule of a perfect gas =32kT
∴ Average KE of molecules of the first gas 32n1kT1
Average KE of molecules of the second gas =32n2kT2
∴ Total KE of the molecules of the two gases before they are mixed is
K=32n1kT1+32n2kT2 =32(n1T1+n2T2)k (1)
If T is the temperature of the mixture, the kinetic energy of the molecules (n1+n2) in the mixture is
K′=32(n1+n2)kT (2)
Since there is no loss of energy K=K'. Equating (1) and (2) we get
n1T1+n2T2=(n1+n2)T or T=n1T1+n2T2n1+n2