Question

solve question no. 29

Answer 1

let initial pressure = P°
after dissolving non volatile solute then pressure = P° - P°×2.5/100
=P° - P°/40
=39P°/40

now use , Raults law,
(P° - P)/P° = mole fraction of solute

(P° -39P°/40)/P° = M/90/( M/90 + 97.5/18)

1/40 = M/90/( M/90 + 97.5/18)

M/90 + 97.5/18 = 40M/90

39M/90 = 97.5/18

39M/5 = 97.5

M = 5×97.5/39

M = 12.5 g