The vapour pressure of a solution of 5g of non electrolyte in 100g of water at a particular temperature is 2 2985 Nm-2 . The vapour pressure of pure water at that temperature is 3000 Nm-2.The molecular weight of the solute is

  1. A

    180

  2. B

    90

  3. C

    270

  4. D

    200

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    Solution:

    Vapour pressure of solution=P=2985Nm-2

    Vapour pressure of water=P0=300Nm-2

    Wt. of non electrolyte solute=W1=5g

    Wt. of water=W2=100g

    GMW of water=M2=18g

    GMW of solute=M1=?

    According to Raoult’s law

    P0-PP0=W1M1W1M1+W2M2

    3000-29853000=5M15M1+10018

    153000=5M190+100M118M1

    153000=5M1×18M1(90+100M1)

    90+100M1=5×18×300015

    100M1=15000×1815-90

    100M1=18000-90

    M1=17910100

    M1=179.1(Near By)

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