Question

calculate the root mean square velocity of F2,Cl2 and Br2 at 302k.

Answer 1

You can derive an expression for the room mean square speed of the molecules only depending on their molecular mass and temperature from the maxwell-boltzmann distribution (among others). The relation is given by:
\[\large v=\left( \frac{ 3RT }{ M } \right)^{1/2}\]
\(R\) is the gas constant, \(T\) the absolute temperature and \(M\) the molar mass.
The equation builds on the approximation that the gas behaves as a perfect gas.