Chemistry · Colligative properties of dilute solutions: a relative lowering of vapour pressure, depression of freezing point, the elevation of boiling point and osmotic pressure
The mass of non-volatile, non-elctrolyte solute (molar mass mol ) needed to be d
The mass of non-volatile, non-elctrolyte solute (molar mass \( 50 g \) mol \( ^{-1} \) ) needed to be dissolved in \( 114 g \) octane to reduce its vapour pressure to \( 75 \% \), is
- A. \( 150 g \)
- B. \( 75 g \)
- C. \( 37.5 g \)
- D. \( 50 g \)
Step-by-step solution
Using Raoult's law, relative lowering of vapour pressure equals mole fraction of solute. If vapour pressure is reduced to 25% (i.e., by 75%), then (P° - P)/P° = 0.75 = x_solute. Moles of octane = 114 g / 114 g/mol = 1 mol. Let mass of solute = w g, moles = w/50. Then x_solute = (w/50) / (w/50 + 1) = 0.75. Solving gives w = 150 g.
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