Question

Which of the following gasses will have the lowest velocity at 200k

Rn

Kr

H2

He

Answer 1

vrms = sqrt (3*R*T/M)

here : T is temperature in Kelvin

and M is molar mass in Kg/mol

Since T is same for all, the one with lower molar mass will have higher speed

Lets list out molar mass of all given compounds

Molar mass of Rn = 222.0

Molar mass of Kr = 83.8

Molar mass of H2 = 2.016

Molar mass of He = 4.003

Increasing order of molar mass is:

H2

He

Kr

Rn

Same is the order of decreasing Vrms

From fastest to slowest

H2

He

Kr

Rn

Answer: Rn will have lowest speed