JEE Main · 2007 · 29 Apr 2007 · Offline · Solutions

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5.25%

solution of a substance is isotonic with a

1.5%

solution of urea (molar mass

= 60 g mol^{- 1}

) in the same solvent. If the densities of both the solutions are assumed to be equal to

1.0 g cm^{- 3}

, molar mass of the substance will be

(A) $90.0 g mol^{- 1}$
(B) $115.0 g mol^{- 1}$
(C) $105.0 g mol^{- 1}$
(D) $210.0 g mol^{- 1}$ ✓ Correct

Answer: (D)

Solution

Isotonic solutions have equal osmotic pressure and therefore equal molar concentration. With the densities taken equal, the mass percentages compare directly, so

\frac{5.25}{M} = \frac{1.5}{60}

for the unknown molar mass

M

. Solving gives

M = \frac{5.25 \times 60}{1.5} = 210

gram per mole.

JEE Main · 2007 · 29 Apr 2007 · Offline · Single correct option

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