Explain why there is a rise in boiling point when a non-volatile solid is dissolved in a liquid. 0.90 g of a non-electrolyte was dissolved in 87.90 g of benzene. This raised the boiling point of benzene by 0.25°C. If the molecular mass of the non-electrolyte is 103 g mol, calculate the molal elevation constant for benzene.
Answer: The solute lowers the vapour pressure, so a higher temperature is needed for it to reach 1 atm. K_b ≈ 2.52 K kg mol⁻¹.
Solution
A non-volatile solute lowers the vapour pressure of the liquid; the solution must therefore be heated to a higher temperature before its vapour pressure equals the external pressure — the boiling point rises.
Molality of the solution:
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Molality of the solution:
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CBSE Class 12 Board · 2003 · Delhi · Subjective