Question

A sample of unknown gas diffuses at half the rate of helium. What is the density of the unknown gas at stp?

The answer is 0.71 g/L.

~I know that you need to use the effusion equation and d=PM/RT, however I seem to either be confused or keep making a simple error in the middle. Thank you

Answer 1

The rate of helium = r

The rate of unknown gas = r/2

r1/r2 = M2/M1

r He/r un = M Un/M He

2/1         = M un/4

4/1    = M Un/4

M    = 16g/mole

molar mass of unknown gas   = 16g/mole

d=PM/RT

At STP     P = 1atm

                 T   = 273K

                 M   = 16g/mole

     d   = 1*16/0.0821*273    = 0.71g/L

The density of unknown gas = 0.71g/L >>>>>>answer