A sample of He is placed on the left side of a porous barrier in a container and
ID: 917824 • Letter: A
Question
A sample of He is placed on the left side of a porous barrier in a container and is found to have a pressure of 0.155atm. After 10.0minutes the pressure of He on the right is 0.112atm. When a sample of an unknown gas at the same temperature and pressure is allowed to diffuse similarily, the pressure on the right side after 10.0 minutes is found to be 0.0264atm. Calculate the molecular weight of the unknown gas. A sample of He is placed on the left side of a porous barrier in a container and is found to have a pressure of 0.155atm. After 10.0minutes the pressure of He on the right is 0.112atm. When a sample of an unknown gas at the same temperature and pressure is allowed to diffuse similarily, the pressure on the right side after 10.0 minutes is found to be 0.0264atm. Calculate the molecular weight of the unknown gas.Explanation / Answer
Graham's law states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight.
Rate = k / sqrt (molecular weight)
k = constant
=> rate x sqrt(molecular weight) = k = constant
For Helium
Rate= 0.112 / 10 = 0.0112 atm min-1
molecular weight = 4
For unknown gas
Rate = 0.0264 / 10 = 0.00264 atm min-1
Let molecular weight = m
=> 0.0112 x sqrt (4) = 0.00264 x sqrt (m)
=> 0.0112 x 2 / 0.00264 = sqrt (m)
=> m = 72 g / mol = Molecular weight of the unknown gas