A teacher enters a classroom from front door while a student from back door. There are 13 equidistant rows of benches in the classroom. The teacher releases `N_(2)O`, the laughing gas, from the first bench while the student releases the weeping gas `(C_(6)H_(11)OBr)` from the last bench. At which row will the students starts laughing and weeping simultaneously?

Answer

`9` th row

Explanation

Molar mass of `C_(6)H_(11)Obr = 6 xx 12 + 1 xx 11 + 16 + 80 = 179 g"mol"^(-1)`
Molar mass of `N_(2)O = 2 xx 14 + 16 = 44 g "mol"^(-1)`
By applying Graham's Law of duffsion,
`(rN_(2)O)/(rC_(6)H_(11)OBr) = sqrt((M_(C_(6)Hl_(11)OBr))/(M_(N_(2)O)))=sqrt((179)/(44))`
Let x be the row at which the student start smetting both the gaseous simultaneously.
`(x)/(13-x) = sqrt((179)/(44)) = 2.0169` or `x = 8.691 ~~ 9`.
This means that at the 9th row, the student will start smelling both the gases.