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What is relative lowering of vapour pres...

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

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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))`
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What is the relative lowering of vapour pressure ? How is it useful in determining the molecular weight of solute ?

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Knowledge Check

  • Which has greater lowering of vapour pressure

    A
    0.1m Urea
    B
    0.1m Glucose
    C
    0.1m Sucrose
    D
    equal in all cases

  • The relative lowering of vapour pressure is equal to

    A
    Ratio of the number solute molecules to the totalnumber of molecules in solution
    B
    Ratio of the number of solvent molecules to the number of solute molecules
    C
    Ratio of the number of solute molecules to the number of solvent molecules
    D
    Ratio of the number of solvent molecules to the total number of molecules in solution

  • Relative lowering of vapour pressure is maximum for

    A
    0.1 m urea
    B
    0.1 m NaCl
    C
    0.1 m `MgCl_2`
    D
    0.1 m `Al_2(SO_4)_3`

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