It took 4.5 minutes for 1.0 l helium to effuse through a porous barrier. how long will it take for 1.0 l cl2 gas to effuse under identical conditions?

When The rate of effusion is inversely proportional to the √molar mass of the substance.

and we have R (He) = 1L / 4.5 min so,

R (He) / R (Cl2) = (molar mass of Cl2 / molar mass of He) ^0.5

and when we have the molar mass of Cl2 = 70.9 & the molar mass of He = 4

so by substitution:

(1L/4.5 min) / R (Cl2) = (70.9 / 4) ^0.5

(1L/4.5 min) / R (Cl2) = 4.21

∴R (Cl2) = (1L/4.5 min) / 4.21 = 1L / (4.5*4.21) min = 1 L / 18.945 min

∴Cl2 will take 18.945 min for 1 L to effuse under identical conditions