Q. A solution of sucrose (Molar mass = 342 g/mol) is prepared by dissolving 68.4 g of it per litre of solution, what is its osmotic pressure (R=0.082 L atm$K^{- 1}\ mol^{- 1}$) at 273 K?
A) 0.01 M $Na_{2}SO_{4}$
B) 0.01 M $KNO_{3}$
C) 0.015 M urea
D) 0.015 M glucose
Explanation
(a)
Boiling point
=$T_{0}(solvent) + \mathrm{\Delta}T_{b}$(Elevation in b.p.)
$\mathrm{\Delta}T_{b} = mik_{b}$
where, m is the molality
i.e., the van’t Hoff factor (i)
=[1+(y-1)x]
$\ \ \ \ \ \ \ \ \ k_{b} = molal\$elevation constant.
Thus, $\mathrm{\Delta}T_{b}\ \ \propto \ im$
Assume 100% ionisation
(a) $mi(Na_{2}SO_{4}) = 0.01 \times 3 = 0.03\ \ \ \ \$
(b) $mi(KNO_{3}) = 0.01 \times 2 = 0.02$
(c) mi(urea)=0.015
(d) mi(glucose)=0.015
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.