At 70°C the vapor pressure of pure water is 31 kPa. Which of the following is most likely the vapor pressure of a 2.0 molal aq. glucose solution at 70°C?(a) 30.001 kPa(b) 29.915 kPa(c) 28.226 kPa(d) 32.392 kPaThis question was addressed to me in class test.Query is from Colligative Properties and Determination of Molar Mass topic in portion Solutions of Chemistry – Class 12

At 70°C the vapor pressure of pure water is 31 kPa. Which of the foll

1.	At 70°C the vapor pressure of pure water is 31 kPa. Which of the following is most likely the vapor pressure of a 2.0 molal aq. glucose solution at 70°C?(a) 30.001 kPa(b) 29.915 kPa(c) 28.226 kPa(d) 32.392 kPaThis question was addressed to me in class test.Query is from Colligative Properties and Determination of Molar Mass topic in portion Solutions of Chemistry – Class 12
Answer» The correct option is (b) 29.915 kPa The best explanation: Given, P^0water = 31 kPa Concentration of solution, C = 2 molal = 2 moles of glucose/KG of water From LAW of relative LOWERING of vapor pressure, ΔP/P^0 = X2, where X2 is the mole fraction of glucose in the solution. Mass of water = 1 kg = 1000 g Molecular weight of water = 18 g/mole Moles of water = 1000/18 = 55.556 moles X2 = 2/(2 + 55.556) = 0.035 ΔP = 31 kPa x 0.035 = 1.085 kPa Final pressure = 31 kPa – 1.085 kPa = 29.915 kPa.

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