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NEET-XII-Chemistry

02: Solutions

with Solutions - page 3
Qstn# B-19 Prvs-QstnNext-Qstn
  • #19
    A solution
    containing 30 g of non-volatile solute exactly in 90 g of water has a
    vapour
    pressure of 2.8 kPa at 298 K.
    Further, 18 g of water is then added to
    the
    solution and the new vapour pressure becomes 2.9 kPa at 298 K.
    Calculate:
    1. molar
      mass of the solute
    2. vapour
      pressure of water at 298 K.
    Ans : (i) Let,
    the molar mass of the solute be M g mol-1
    Now,
    the no. of moles of solvent (water),
    And,
    the no. of moles of solute,

    Applying the relation:

    After
    the addition of 18 g of water:

    Again, applying the relation:

    Dividing
    equation (i) by (ii),
    we have:

    Therefore,
    the molar mass of the solute is 23 g mol-1.
    (ii) Putting
    the value of ‘M’ in equation (i),
    we have:

    Hence, the vapour pressure of water at 298 K is 3.53 kPa.