KVPY Sample Paper KVPY Stream-SX Model Paper-26

Pure water freezes at 273 K and 1 bar. The addition of 34.5 g of ethanol to 500 g of water changes the freezing point of the solution. Use the freezing point depression constant of water as 2 K kg \[{{\operatorname{mol}}^{-1}}\]. The figures shown below represent plots of vapour pressure (V.P.) versus temperature (T). molecular weight of ethanol is 46 g \[{{\operatorname{mol}}^{-1}}\]] Among the following, the option representing change in the freezing point is

A)

B)

C)

D)

Correct Answer: C

Solution :

As T increase, V.P. increase \[\Delta {{T}_{f}}={{K}_{f}}\times m\] (m=molality of solute)

\[273-{{T}_{f}}'=2\times \frac{34.5\times 1000}{46\times 500}\]

\[\therefore \] \[{{T}_{f}}'=270K\]

Thus freezing point of solution = 270K. Further as T increases, vapour pressure increases. Hence these coincide with the curve gives in [c].