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NEET-XII-Chemistry

02: Solutions

with Solutions - page 3
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  • #21
    Two
    elements A and B form compounds having formula AB2 and AB4.
    When dissolved in 20 g of benzene (C6H6),
    1 g of AB2 lowers the freezing point by 2.3 Kwhereas 1.0 g of AB4 lowers it by 1.3 K.
    The molar depression constant for benzene is 5.1 Kkg mol-1.
    Calculate atomic masses of A and B.
    Ans : We know that,

    Then,
    = 110.87 g mol-1

    = 196.15 g mol-1
    Now, we have the molar
    masses of AB2 and AB4 as 110.87 g mol-1 and 196.15 g mol-1 respectively.
    Let
    the atomic masses of A and B be x and y respectively.
    Now,
    we can write:

    Subtracting equation
    (i) from (ii), we have
    2y = 85.28
    ⇒ y =
    42.64
    Putting the value of
    ‘y’ in equation (1), we have
    x + 2 ×
    42.64 = 110.87
    ⇒ x =
    25.59
    Hence, the atomic
    masses of A and B are 25.59 u and 42.64 u respectively.
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