The average molecular speed in a sample of NO2 gas at a certain temperature is 374 m/s. The average molecular speed in a sample of Kr gas is m/s at the same temperature. The rate of effusion of Kr gas through a porous barrier is observed to be 8.35 x 10-4 mol/h. Under the same conditions, the rate of effusion of CH4 gas would be mol/h. A sample of Kr gas is observed to effuse through a pourous barrier in 5.11 minutes. Under the same conditions, the same number of moles of an unknown gas requires 3.16 minutes to effuse through the same barrier. The molar mass of the unknown gas is g/mol.