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The vapour pressure of an aqueous solution of sucrose at 373 K is found to be 750 mm Hg. The molality of the solution at the same temperature will be:
  • a)
    0.26
  • b)
    0.73
  • c)
    0.74
  • d)
    0.039
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
The vapour pressure of an aqueous solution of sucrose at 373 K is foun...
At 373K V.P of water is 750 mm Hg

We know that, (P0-P) / P0 =  w x M / m x W

molality = (P0-P) / P0 X 1000/M 

=> (760-750) / 760 X 1000/18

=> 0.73 m

Mole fraction = (P0-P) / P0  = 10/760 = 0.0131  
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The vapour pressure of an aqueous solution of sucrose at 373 K is foun...
Given:
Vapour pressure of aqueous solution of sucrose at 373 K = 750 mm Hg
To find: Molality of the solution at the same temperature

Formula:
The relation between vapour pressure and molality of the solution is given by the formula:
ΔP = P° - P
ΔP = Kf × b × i
Where,
ΔP = depression in vapour pressure
P° = vapour pressure of the pure solvent
P = vapour pressure of the solution
Kf = molal depression constant
b = molality of the solution
i = van’t Hoff factor

Calculation:
Given that the vapour pressure of the solution is 750 mm Hg.
At 373 K, the vapour pressure of water is 760 mm Hg.
Therefore, the depression in vapour pressure can be calculated as:
ΔP = P° - P = 760 - 750 = 10 mm Hg

The molal depression constant (Kf) for water is 1.86 K kg mol⁻¹.
The van’t Hoff factor (i) for sucrose is 1 (since it does not dissociate in water).

Substituting the values in the formula:
ΔP = Kf × b × i
10 = 1.86 × b × 1
b = 10/1.86 = 5.376 mol kg⁻¹

Therefore, the molality of the solution at 373 K is 0.74 mol kg⁻¹ (rounded off to two decimal places).

Hence, the correct option is (C) 0.74.
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The vapour pressure of an aqueous solution of sucrose at 373 K is found to be 750 mm Hg. The molality of the solution at the same temperature will be:a)0.26b)0.73c)0.74d)0.039Correct answer is option 'C'. Can you explain this answer?
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