5 g of urea is dissolved in one kg of water. Up to what temperature the solution may be cooled before ice starts crystallizing out? ( of water = 1.86)

Question

5 g of urea is dissolved in one kg of water. Up to what temperature the solution may be cooled before ice starts crystallizing out? ($K_f$ of water = 1.86)

Prashant Kotian · Accepted Answer

Correct answer: D. Molar mass of urea (NH2CONH2) = 60 g/mol. Moles of urea = 5 / 60 = 0.0833 mol. Molality m = 0.0833 mol / 1 kg = 0.0833 m. Depression in freezing point ΔT_f = * m = 1.86 * 0.0833 ≈ 0.155°C. Since pure water freezes at 0°C, ice will start to crystallize out at -0.155°C.

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