a) Define the following terms:

(i) Mole fraction

(ii) Ideal solution

(b) 15.0 g of an unknown molecular material is dissolved in 450 g of water. The resulting solution freezes at - 0.34°C. What is the molar mass of the material? (Kf for water = 1.86 K kg mol-1)


 (i) Mole fraction: The mole fraction of a component in a mixture is defined as the ratio of the number of moles of the component to the total number of moles of all the components in the mixture. Mathematically, it is represented as:

 Mole straight space fraction straight space of straight space straight a straight space component straight space equals fraction numerator Number straight space of straight space moles straight space of straight space the straight space component straight space over denominator total straight space number straight space of straight space moles straight space of straight space all straight space components end fraction

 Mole fraction is denoted by ‘x’.



(ii) Ideal Solution:

Solutions which obey Raoult’s law over the entire range of concentrations are known as ideal solution. Along with that for ideal solution:

Enthalpy of mixing of the pure components to form the solution i.e mix H = 0 and volume of mixing, mix V = 0.

An ideal solution will be formed when intermolecular forces of attraction between the molecules of solute (A - A) and those between the molecules of solvent (B -B) are nearly equal to those between solute and solvent molecules (A - B).

For Example: n-Hexane and n-heptane


(b) Given mass of solute = wb = 15.0g

Molar mass of solute = Mb =?

Mass of water = wa = 450 g

Freezing point of water = 0°C = 273 K

Freezing point of solution = - 0.34°K

= (- 0.34 + 273) K

Depression in freezing point = Tf= 273 - (-0.34 + 273)

= 0.34 K

Kf for water = 1.86 K Kg mol-1

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