What is relative lowering of vapour pressure ? How is it useful to determine the molar mass of a solute ?

Explanation

Raoult's law for volatile solute : For a solution of volatile liquids, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in solution.
Raoult's law for non-volatile solute : The relative lowering of vapour pressure of dilute solution containing non-volatile solute is equal to the mole fraction of solute.
Relative lowering of vapour pressure `(P_(0)-P_(s))/(P_(0))=X_(s)` (mole fraction of solute)
`(P_(0)-P_(s))/(P_(0))=(n_(s))/(n_(0)+n_(s))`
For very much dilute solutions `n_(s) lt lt lt ...... n_(0)`
`:.(P_(0)-P_(s))/(P_(0))=(n_(s))/(n_(0))=("w")/("m")xx("M")/("W")`
W = Weight of solue
w = Weight of solvent
m = Molar mass of solute
M = Molar mass of solvent
Molar mass of solute `m=("w"xx"M")/("W")xx(P_(0))/(P_(0)-P_(s))`