Question
(NEET 2022 PYQ) The boiling point of pure water is 100°C. When 36 g of glucose ( g/mol) is dissolved in 500 g of water, the boiling point increases. Find the new boiling point. Given of water K kg/mol.
Solution — Step by Step
Molality = (moles of solute) / (kg of solvent).
Moles of glucose = mol.
Mass of water = 500 g = 0.5 kg.
For non-electrolytes like glucose, the van’t Hoff factor , so no correction needed.
New boiling point ≈ 100.21°C.
Why This Works
Adding a non-volatile solute lowers the vapour pressure of the solvent, so the solution must be heated higher to reach 1 atm vapour pressure. The elevation is proportional to the molality of the dissolved particles — this is a colligative property.
For ionic solutes (NaCl, MgCl₂), the van’t Hoff factor accounts for dissociation. NaCl gives (Na⁺ + Cl⁻), MgCl₂ gives . So .
NEET frequently tests “which solution has highest boiling point” — answer is the one with maximum . For 0.1 M solutions: glucose (), NaCl (), CaCl₂ (), Al₂(SO₄)₃ (). Highest = Al₂(SO₄)₃.
Alternative Method
Direct calculation from mole fraction and Raoult’s law: . Then convert vapour pressure change to boiling point change using Clausius-Clapeyron. Much longer — is the shortcut for dilute solutions.
Common Mistake
Students confuse molality (mol/kg solvent) with molarity (mol/L solution). For colligative properties, always use molality — it’s temperature-independent. Computing molarity from mass percent gives a wrong answer.