Solutions

Solutions

Question

a) Calculate the freezing point of the solution when 1.9 g of MgCl2 (M = 95 g mol−1) was dissolved in 50 g of water, assuming MgCl2 undergoes complete ionization.
 (Kf for water = 1.86 K kg mol−1)

(b)
(i) Out of 1 M glucose and 2 M glucose, which one has a higher boiling point and why?
(ii) What happens when the external pressure applied becomes more than the osmotic pressure of solution?

OR

(a)When 2.56 g of sulphur was dissolved in 100 g of CS2, the freezing point lowered by 0.383 K. Calculate the formula of sulphur (Sx).
 
(Kf for CS2 = 3.83 K kg mol−1, Atomic mass of sulphur = 32 g mol−1]

(b)Blood cells are isotonic with 0.9% sodium chloride solution. What happens if we place blood cells in a solution containing
(i)1.2% sodium chloride solution?
(ii)0.4% sodium chloride solution?

Answer
straight a right parenthesis space Given colon
straight K subscript straight f space equals space 1.86 space straight K space kg space mol to the power of negative 1 end exponent
Mass space of space solute space equals space 1.9 space straight g
Mass space of space solvent space equals space 50 straight g
Therefore comma
Molality space of space the space solution comma space straight m space equals space straight m equals space fraction numerator 1.9 space over denominator 95 end fraction straight x space 1000 over 50
straight m equals space 0.4 straight m

Also comma space MgCl subscript 2 space undergoes space complete space ionisation space and space there space by space yielding space 3 space moles space of space
constituent space ions space for space every space mole space of space MgCl subscript 2.
straight i equals 3
Now comma space depression space in space freezing space point space given space as
increment straight T subscript straight f space equals space iK subscript straight f straight m
equals 3 space straight x space 1.86 space straight x space 0.4
space equals 2.232 space straight K
straight T subscript straight f equals 273.15 minus 2.232 equals 270.918 space straight K
Hence comma space the space new space freezing space point space of space the space solution space is space 20.92 space straight K.

(i) The elevation in the boiling point of a solution is a colligative property; therefore, it is affected by the number of particles of the solute. Since the amount of solute is higher in 2 M glucose solution as compared to 1 M glucose solution, the elevation in the boiling point is higher. Hence, 2 M glucose solution has a higher boiling point than 1 M glucose solution.

(ii) When the external pressure exerted on the solution is higher than the osmotic pressure, the pure solvent starts flowing out of the solution through the semi- permeable membrane. This process is known as reverse-osmosis.
Or
Given colon
straight K subscript straight f space equals space 3.83 straight K space kg space mol to the power of negative 1 end exponent
Mass space of space solute space equals space 2.56 space straight g
Mass space of space solvent space equals space 100 straight g
Therefore comma
Molality space of space the space solution space comma space straight m space equals fraction numerator 2.56 over denominator 32 end fraction space straight x space 1000 over 100
straight m space equals space 0.8 space straight m

The space depression space in space freezing space point space of space straight a space solution space is space given space as
increment straight T subscript straight f space equals space iK subscript straight f straight m
0.383 space equals space straight i space straight x space 3.83 space straight x space 0.8
straight i space equals 1 over 8
Hence comma space 8 space sulphur space atoms space are space undergoing space association space as space shown space below colon
8 straight S rightwards harpoon over leftwards harpoon straight S subscript 8

(b)
(i) 1.2% sodium chloride solution is hypertonic wrt 0.9% sodium chloride solution. Therefore, when the blood cells are placed in 1.2% sodium chloride solution, water flows out of the cells and the cells shrink.

(ii) 0.4% sodium chloride solution is hypotonic wrt 0.9% sodium chloride solution. Therefore, when the blood cells are placed in 0.4% sodium chloride solution, water flows into the cells and the cells swell.

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