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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3so150

A $2.00 m$ aqueous solution of urea (molar mass $60 g mo l^{- 1}$ ) has density $1.03 g m L^{- 1}$ (water $18 g mo l^{- 1}$ ). Which statements are correct?

AThe molarity of the solution is 1.84 M.
BThe molarity of the solution is 2.06 M.
CThe mole fraction of urea is about 0.035.
DThe mass percentage of urea is 12.0%.

Answer: A, C

Per kg of water there are 2.00 mol urea (

120

g), so

x = 2.00/ (2.00 + 55.56) = 0.0347

, about

0.035

. Mass percent

= 120/1120 \times 100 = 10.7%

, not 12.0%. Solution volume

= 1120/1.03 = 1087

mL, so

M = 2.00/1.087 = 1.84

M; the value 2.06 M wrongly multiplies molality by density.

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