18g glucose is dissolved in 90g water the relative lowering in vapour pressure is equal to ___________________?
Correct answer: D. one/fifty one
- A. 8
- B. 5.1
- C. 6
- D. one/fifty one
Explanation
For a dilute solution of a non-volatile solute, relative lowering of vapour pressure equals the solute mole fraction. Glucose moles = 18 g / 180 g mol-1 = 0.1 mol, and water moles = 90 g / 18 g mol-1 = 5.0 mol, so xsolute = 0.1 mol / (0.1 mol + 5.0 mol) = 0.1/5.1 = 1/51. Students may choose 1/50 by using only the solvent moles in the denominator instead of total solution moles.
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About Evaporation, Boiling Point and Vapour Pressure
Evaporation is the escape of higher-energy molecules from a liquid surface, while vapour pressure is the pressure exerted by vapour in equilibrium with that liquid. The topic distinguishes evaporation from boiling and relates both to temperature, intermolecular forces, surface area, atmospheric pressure and the boiling point.
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