A gas mixture consists of molecules of A, B and C with masses mA>mB>mC. Rank the three types of molecules in decreasing order of (a) average KE (b) RMS speeds.

Hint: The KE depends on the temperature and the RMS speed depends on the molar mass.
(a) Step 1: Find the ratio of the KE.
The average KE will be the same, as conditions of temperature and pressure are the same.
Step 2: Find the ratio of the RMS speed.
(b) Now as,
                  vrms=3pVM=3RTM
       =3RTmN=3kTm
where M =molar mass of the gas
 m=mass of each molecule of the gas,
R = gas constant
Clearly,                  vrms1m
As k = Boltzmann constant, T = absolute temperature (same for all)
But,                                 mA>mB>mC
                                          (vrms)A<(vrms)B<(vrms)C
                                       (vrms)C>(vrms)B>(vrms)A