A mixture of `O_(2)` and gas `"Y"` `( mol. wt. 80)` in the mole ratio `a:b` has a mean molecular weight 40. What would be mean molecular weight, if the gases are mixed in the ratio `b:a` under identical conditions ? ( gases are )

Options

A. 40
B. 48
C. 62
D. 72

Answer

D

Explanation

Let mole fraction of `O_(2)` is x
`40=32 xx x + 80 (1-x)` or `|x = 5 // 6`
 a: b=x:(1-x)= `(5)/(6):(1)/(6)`
When ratio is changed
`M_("mixture") = 32 xx (1)/(60) +80 xx (5)/(6) = 72`