A flask contains argon and chlorine in the ratio of 2 : 1 by mass. The temperature of the mixture is 27°C. The ratio of root mean square speed of the molecules of the two gases ($v_{rms}(Ar) / v_{rms}(Cl_2)$) is: (Atomic mass of argon = 40.0 u and molecular mass of chlorine = 70.0 u) ____.