1 litre of oxygen effuses through a small hole in 60 min. and a litre of helium at the same temperature and pressure effuses through the same hole in 21.2 min. What is the atomic weight of Helium -

Options

A. 2.99
B. 3.99
C. 2.08
D. 1.99

Answer

B

Explanation

`(r_(O_(2)))/( r_(He)) = ( 1000//60)/( 1000// 212) = ( 21.2)/( 60) = sqrt((M_(He))/(M_(O_(2)))) =sqrt( (M_(He))/( 32))`
Squaring both of sides `= ( ( 21.2)^(2))/( ( 60)^(2)) = ( M_(He))/( 32) `
`M_(He) = ((21.2)^(2) xx 32)/( ( 60)^(2)) = 3.99`
Since Helium is monoatomic so
Atomic weight = Molecular weight = 3.99