Q.(a) What do you mean by elevation of boiling point ? Derive the relationship between elevation of boiling point and molecular mass of solute. [2+3=5]

Showing the 12 most recent of 18 on this concept.

1. CBSE 2026Set A1 markMCQ
   Q.The elevation in boiling point produced by one molal solution of a solute in a solvent is called

   (a) Ebullioscopic constant

   (b) Vapour pressure constant

   (c) Cryoscopic constant

   (d) None of these

   ›Reveal solutionSolution

   The boiling-point elevation for a 1 molal solution equals Kb, the molal elevation (ebullioscopic) constant.

   The elevation in boiling point is ΔTb = Kb x m. When molality m = 1, ΔTb = Kb, called the ebullioscopic constant (molal elevation constant). The cry …

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2. CBSE 2026Set ANNUAL1 mark
   Q.The unit of molal elevation constant is ______ (fill in the blank).

   ›Reveal solutionSolution

   The molal elevation constant Kb is defined by ΔTb = Kb·m, so its unit follows from rearranging: Kb = ΔTb/m.

   …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.Which of the following aqueous solutions will have the maximum boiling point?

   (a) 1% glucose

   (b) 1% sucrose

   (c) 1% NaCl

   (d) 1% CaCl2

   ›Reveal solutionSolution

   Boiling point elevation, delta-Tb = i x Kb x m, depends on the TOTAL number of solute particles, not the solute's identity - so for equal 1% mass solutions we must compare (mass/molar mass) x i for each.

   For a fixed mass percentage (1% w/V), the effective molal concentration driving boiling-point elevation is proportional to (1/M) x i, where M is the molar mass and i is the van't Hoff factor (number of particles the solute dissociates into).

   • 1% glucose (M = 180 g/mol, i = 1): (1/180) x 1 = 0.00556
   • 1% sucrose (M = 342 g/mol, i = 1): (1/342) x 1 = 0.00292 …

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4. CBSE 2026Set SEM31 markMCQ
   Q.If the elevation in boiling point of a solution of 10 g of non-volatile, non-dissociative, non-associative solute in 100 g of water is dTb and if molar mass of the solute is 100 then Ebullioscopic constant of water 'Kb' is

   (a) 100 (dTb)

   (b) dTb

   (c) dTb / 100

   (d) 10 (dTb)

   ›Reveal solutionSolution

   Elevation in boiling point dTb = Kb x m. Here molality m = 1 mol/kg, so Kb = dTb. Correct option (b).

   For a dilute solution of a non-volatile, non-dissociating solute: dTb = Kb x m, where m is molality.

   Step 1 - moles of solute: n = mass / molar mass = 10 / 100 = 0.1 mol.

   Step 2 - mass of solvent: 100 g water = 0.1 kg.

   Step 3 - molality: m = n / (kg solvent) = 0.1 / 0.1 = 1 mol/kg. …

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5. CBSE 2025Set D1 markMCQ
   Q.Which among the following aqueous solutions has the highest boiling point?

   (a) 1% glucose

   (b) 1% sucrose

   (c) 1% NaCl

   (d) 1% CaCl2

   ›Reveal solutionSolution

   Boiling-point elevation depends on total particle concentration; 1% NaCl gives the most particles, so it has the highest boiling point.

   Elevation of boiling point is a colligative property: delta Tb = i x Kb x molality, so the solution with the largest (i x moles of particles) for the same mass wins. For 1% w/w (same mass of solute per unit solvent), compute particles per gram = (i / molar mass):

   • 1% glucose: non-electrolyte i = 1, M = 180 -> 1/180 = 0.0056
   • 1% sucrose: non-electrolyte i = 1, M = 342 -> 1/342 = 0.0029
   • 1% NaCl: i = 2, M = 58.5 -> 2/58.5 = 0.0342 …

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6. CBSE 2025Set ANNUAL1 markMCQ
   Q.Which of the following aqueous solutions should have the highest boiling point?

   (a) 1.0 M NaOH

   (b) 1.0 M Na2SO4

   (c) 1.0 M NH4NO3

   (d) 1.0 M KNO3

   ›Reveal solutionSolution

   Boiling point elevation is a colligative property that depends on the total number of solute particles in solution, not their identity - so at equal molar concentration, the electrolyte that dissociates into the most ions gives the highest boiling point.

   Elevation in boiling point: $\Delta T_b = i K_b m$, where $i$ is the van't Hoff factor (number of particles produced per formula unit on dissociation).

   At the same concentration (1.0 M) for all four solutes, compare $i$:

   • NaOH -> Na+ + OH- : i = 2
   • Na2SO4 -> 2Na+ + SO4^2- : i = 3
   • NH4NO3 -> NH4+ + NO3- : i = 2 …

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7. CBSE 2025Set ANNUAL1 markMCQ
   Q.An example of colligative property of solution is

   (a) density

   (b) mass

   (c) elevation of boiling point

   (d) temperature

   ›Reveal solutionSolution

   Colligative properties depend only on the number of solute particles present, not on their chemical nature.

   Density, mass and temperature are ordinary physical quantities that do not depend on the number of dissolved particles - they are not colligative properties. Elevation of boiling point, however, depends directly on the mole fraction/molality of the solute particles (via $\Delta T_b = i K_b m$), making …

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8. CBSE 2025Set ANNUAL1 mark
   Q.Assertion [A] : On addition of non-volatile solute in a pure solvent, boiling point of the solution increases. Reason [R] : On addition of non-volatile solute in a pure solvent, vapour pressure of solution increases.

   ›Reveal solutionSolution

   Boiling-point elevation is real (Assertion true), but it happens because vapour pressure DECREASES (not increases) when a non-volatile solute is added — so the stated Reason is false.

   By Raoult's law, adding a non-volatile solute lowers the mole fraction of solvent at the surface, which LOWERS the vapour pressure of the solution below that of the pure solvent. Because the solution's vapour pressure is now lower, it must be heated to a HIGHER temperature before its vapour pressure equals atmospheric pressure (i.e., before it boils) — this is exactly why boiling point rises ( …

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9. CBSE 2025Set ANNUAL1 markMCQ
   Q.The units of ebulloscopic constant is:

   (a) K kg mol⁻¹

   (b) mol kg K⁻¹

   (c) kg mol⁻¹ K⁻¹

   (d) K mol kg⁻¹

   ›Reveal solutionSolution

   The ebullioscopic constant (molal elevation constant) $K_b$ has units K kg mol⁻¹.

   Boiling point elevation is given by:

   $\Delta T_b = K_b \times m$

   where $\Delta T_b$ is in kelvin and $m$ (molality) is in mol kg⁻¹. Rearranging, …

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10. CBSE 2025Set ANNUAL1 mark
    Q.Out of two 0.1 molal solution of glucose and of potassium chloride, why do 0.1 molal solution of potasium chloride have a higher boiling point ?

    ›Reveal solutionSolution

    Since boiling-point elevation depends on the number of dissolved particles, and KCl dissociates into two ions per formula unit while glucose stays as one particle, KCl produces roughly double the effect at the same molal concentration.

    Boiling point elevation is a colligative property, given by $\Delta T_b = i K_b m$, where $i$ is the van't Hoff factor (the effective number of particles each formula unit produces in solution).

    • Glucose is a non-electrolyte — it dissolves as intact molecules, so $i \approx 1$.
    • Potassium chloride (KCl) is a strong electrolyte and dissociates essentially completely in water: $KCl \rightarrow K^+ + Cl^-$, giving $i \approx 2$. …

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11. CBSE 2025Set ANNUAL1 mark
    Q.What is ebullioscopic constant?

    ›Reveal solutionSolution

    Kb converts a measured molality of dissolved solute directly into how much the boiling point should rise, ΔTb = Kb × m.

    Boiling-point elevation is a colligative property: ΔTb = Kb × m, where m is the molality of the solution and Kb is a proportionality constant characteristic of the solvent (not the solute), called the ebullioscopic constant or molal elevation constant. By definition, Kb numerically equals the boiling-point elevation that would result from dissolving exactly 1 mole of a non-volatile, non-di …

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12. CBSE 2024Set ANNUAL1 mark
    Q.What is boiling point?

    ›Reveal solutionSolution

    Boiling begins exactly when the internal vapour pressure of the liquid can push back against, i.e. equal, the surrounding atmospheric pressure.

    As a liquid is heated, its vapour pressure rises. Boiling occurs when the vapour pressure becomes equal to the surrounding (atmospheric) pressure, allowing bubbles of vapour to form freely within the bulk liquid (not just evaporate from the surface). The normal boiling point is this temperature when the external pressure is exactly 1 atmosphere (1.013 bar). This concept underlies boiling-point elevation of a solution: a non-volatile solute lowers the solvent's vapour pressure, so a higher t …

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