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The number of moles of solute n may be given as M . Here W2 is the weight of the solute
and M2 is its molar mass
Substituting the value of n in the above expression.
π = VM or M2 =
Thus, the molar mass of the solute, M2, can be calculated.
Explain reverse osmosis.
(a) Definition : If, a solution is separated from the pure solvent by a semi-permeable
membrane and the pressure applied on the solution (more than the osmotic pressure)
the solvent starts flowing from the solution towards the pure solvent. This phenomenon
is known as reverse osmosis.
(b) Application of reverse osmosis :
Desalination :
(1) Sea water contains 3.5% w/w of dissolved salts.
(2) Drinking water is produced from sea water by a process called as desalination by
reverse osmosis.
(3) In reverse osmosis high pressure greater than osmotic pressure of sea water i.e.,
30 atm. is applied to force water from concentrated aqueous solution like sea water
to pure water side through a semi permeable membrane.
(4) The method can be used provided a suitable semipermeable membrane is developed
which withstands the high pressure condition over a prolonged period.
[Refer Fig. 2.4 (h)]
Explain Abnormal molecular masses.
(a) Colligative properties of solutions depend on actual number of solute particles present in
the solution.
(b) In Dilute solution of non-electrolytes like urea, glucose etc. the solute remains in normal
molecular condition and does not undergo dissociation or association.
(c) In case of solutions of electrolytes, it has been observed that, observed colligative properties
and thus, molar mass determined by these methods do not agree with the expected or
theoretical values.
(d) This is because of association or dissociation of the solute molecules in the solution.
(1) Dissociation :
(i) Molecules of certain substances (acids, bases and salts) dissociate or ionize in
a solvent to give two or more particles. For example, AB dissociates to give
double number of particles as :
A + + B –
AB Chapter - 2 Solutions and Colligative Properties
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