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 kPab)29.915 kPac)28.226 kPad)32.392 kPaCorrect answer is option 'B'. Can you explain this answer?

Question

At 70&deg;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&deg;C?a)30.001 kPab)29.915 kPac)28.226 kPad)32.392 kPaCorrect answer is option 'B'. Can you explain this answer?

Answer

Given, P⁰water = 31 kPa
Concentration of solution, c = 2 molal = 2 moles of glucose/kg of water
From law of relative lowering of vapor pressure, ΔP/P⁰ = 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.

Answer

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