Q.On dissolving 2.5 g of a non-volatile substance in 100 g of benzene, the boiling point rose by 0.4∘C. The molal elevation constant for benzene is 2.67. Calculate the molecular weight of the substance.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Elevation of Boiling Point
Why does salt make water boil hotter?
You already know that pure water boils at 100∘C at 1 atm pressure. But if you dissolve salt or sugar in that water, the boiling point goes up. Not by much — a pinch of salt raises it by a fraction of a degree — but measurably. The question is: why?
Think about what boiling actually is. A liquid boils when its vapour pressure equals the surrounding atmospheric pressure. At that point, bubbles of vapour can form anywhere inside the liquid, not just at the surface. So boiling temperature is really the temperature at which the liquid's vapour pressure hits the external pressure.
Now add a non-volatile solute — something like salt or sugar that does not itself evaporate. The solute particles stay behind in the liquid. They get in the way of solvent molecules trying to escape into the vapour phase. Fewer solvent molecules make it to the surface per second, so the vapour pressure of the solution is lower than that of the pure solvent at the same temperature.
If the vapour pressure is lower, you need to heat the solution to a higher temperature to bring that vapour pressure back up to atmospheric pressure. That higher temperature is the new boiling point. The difference between this new boiling point and the pure solvent's boiling point is the elevation of boiling point, denoted ΔTb.
The solute must be non-volatile. If the solute itself evaporates (like alcohol in water), the reasoning changes completely — both components contribute to vapour pressure.
The precise statement
For dilute solutions of a non-volatile solute, the elevation of boiling point is directly proportional to the molal concentration of the solute. Molality (m) is the number of moles of solute per kilogram of solvent.
ΔTb∝m
Introducing the proportionality constant Kb, called the ebullioscopic constant (or boiling point elevation constant) of the solvent:
ΔTb=Kb⋅m
Here:
- ΔTb is the boiling point elevation (in K or °C — the numerical difference is the same)
- Kb is a property of the solvent alone, not the solute. For water, Kb=0.512 K kg mol−1
- m is the molality of the solution
So if you dissolve 1 mole of a non-volatile solute in 1 kg of water, the boiling point rises by 0.512∘C — from 100∘C to 100.512∘C.
Molality (m) is not the same as molarity (M). Molality uses mass of solvent (kg), molarity uses volume of solution (L). For dilute aqueous solutions they are numerically close, but in exact problems the distinction matters.
Why proportional to molality, not molarity?
Because boiling point elevation depends on the number of solute particles relative to the mass of solvent, not the volume of the solution. Temperature changes affect volume (and therefore molarity), but mass stays constant. Molality is temperature-independent, making it the natural choice for a property that itself depends on temperature.
A concrete example
Suppose you dissolve 58.44 g of NaCl (table salt, molar mass 58.44 g/mol) in 500 g of water. That is 1 mole of NaCl in 0.5 kg of water, so molality m=2 mol/kg.
But NaCl dissociates in water into Na⁺ and Cl⁻ ions — two particles per formula unit. For ionic solutes, the effective number of particles is given by the van't Hoff factor i. For NaCl, i≈2. …
Using ΔTb=MWKbw1000, M=ΔTbWKbw1000=0.4×1002.67×2.5×1000. …
Elevation of boiling point gives M=ΔTbWKbw1000≈166.9 g mol−1.
Concept: Boiling-point elevation is a colligative property: ΔTb=Kbm, where the molality m=M×Ww×1000 (w = mass of solute, W = mass of solvent in g, M = molar mass). Rearranging for M:
M=ΔTbWKbw1000.
…
Showing the 12 most recent of 18 on this concept.
- CBSE 2026Set A1 markMCQQ.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 …
- CBSE 2026Set ANNUAL1 markQ.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.
…
- CBSE 2026Set ANNUAL1 markMCQQ.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 …
- CBSE 2026Set SEM31 markMCQQ.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. …
- CBSE 2025Set D1 markMCQQ.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 …
- CBSE 2025Set ANNUAL1 markMCQQ.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: ΔTb=iKbm, 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 …
- CBSE 2025Set ANNUAL1 markMCQQ.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 ΔTb=iKbm), making …
- CBSE 2025Set ANNUAL1 markQ.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 ( …
- CBSE 2025Set ANNUAL1 markMCQQ.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) Kb has units K kg mol⁻¹.
Boiling point elevation is given by:
ΔTb=Kb×m
where ΔTb is in kelvin and m (molality) is in mol kg⁻¹. Rearranging, …
- CBSE 2025Set ANNUAL1 markQ.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 ΔTb=iKbm, 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≈1.
- Potassium chloride (KCl) is a strong electrolyte and dissociates essentially completely in water: KCl→K++Cl−, giving i≈2. …
- CBSE 2025Set ANNUAL1 markQ.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 …
- CBSE 2024Set ANNUAL1 markQ.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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