The mole fraction of methanol in its 4.5 molal acqueous solution is

Options

A. 0.25
B. 0.125
C. `0.100`
D. `0.075`

Answer

D

Explanation

Let a mole of `CH_(3)OH` and b mole of water be present in the solution .
Mass of water ` = b xx 18`
Molality of solution ` = (a)/(b xx 18) xx 1000 = 4.5`
` or (a)/(b) = (18 xx 4.5)/(1000) = 0.081`
Mole fraction of `CH_(3) OH`
` = (a)/(a+b) or (1)/(1+b//a)`
` = (1)/(1+(1)(0.081)) = (1)/(1+12.34) = 0.075`