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Tip: This calculator is for solutions (solute dissolved in a solvent). For pressure/altitude effects in pure liquids, use your boiling point vs. pressure calculators.

Molality uses kg of solvent (not liters of solution).

Kᵦ is the ebullioscopic constant of the solvent.

Units: °C·kg/mol. If you used a solvent preset, this is auto-filled.

Units: °C. For water at 1 atm, Tᵦ = 100.0°C.

Real solutions can have i < ideal due to ion pairing (especially at higher concentrations).

i = number of dissolved particles per formula unit (idealized).

Units: mol/kg solvent.

Units: °C. If you’re solving forward, we compute ΔTᵦ for you.

Options

Chips prefill values and calculate immediately.

Result

No results yet. Enter values and click Calculate.

How to use this calculator

  • Pick a mode (solve ΔTᵦ, new boiling point, or reverse-solve).
  • Choose a solvent preset (auto-fills Kᵦ and Tᵦ).
  • Set i (electrolyte vs nonelectrolyte) and enter molality m directly — or compute it from masses.
  • Click Calculate to get the answer, steps, and thermometer visual.

How this calculator works

  • Boiling point elevation is a colligative property: ΔTᵦ = i·Kᵦ·m.
  • New boiling point: Tᵦ,new = Tᵦ,normal + ΔTᵦ.
  • Molality from masses: m = (mass/MW) / (kg solvent).

Formula & Equation Used

Boiling point elevation: ΔTᵦ = i·Kᵦ·m

New boiling point: Tᵦ,new = Tᵦ,normal + ΔTᵦ

Molality: m = nsolute / (kg solvent)

Moles from mass: n = (mass in g) / (MW in g/mol)

Example Problem & Step-by-Step Solution

Example 1 — NaCl in water (idealized)

A solution has molality m = 0.75 mol/kg of NaCl in water. Assume ideal dissociation i = 2. For water, Kᵦ = 0.512 °C·kg/mol and Tᵦ,normal = 100.0°C.

  1. Compute elevation: ΔTᵦ = i·Kᵦ·m = 2·0.512·0.75 = 0.768°C
  2. New boiling point: Tᵦ,new = 100.0 + 0.768 = 100.768°C

Note: Real NaCl solutions often have i < 2 depending on concentration.

Example 2 — Sugar in ethanol (nonelectrolyte)

A solution contains a nonelectrolyte (like sugar), so i ≈ 1. The molality is m = 0.30 mol/kg in ethanol. For ethanol, Kᵦ = 1.22 °C·kg/mol and Tᵦ,normal = 78.37°C.

  1. Compute elevation: ΔTᵦ = i·Kᵦ·m = 1·1.22·0.30 = 0.366°C
  2. New boiling point: Tᵦ,new = 78.37 + 0.366 = 78.736°C

This is why the solvent matters: different solvents have different Kᵦ values.

Example 3 — Reverse: solve molality from ΔTᵦ

A solution in water has a measured boiling point elevation of ΔTᵦ = 1.20°C. The solute is a nonelectrolyte, so i = 1. For water, Kᵦ = 0.512 °C·kg/mol and Tᵦ,normal = 100.0°C. Find the molality and the new boiling point.

  1. Start with ΔTᵦ = i·Kᵦ·m
  2. Rearrange: m = ΔTᵦ/(i·Kᵦ)
  3. Plug in: m = 1.20/(1·0.512) = 2.34375 mol/kg
  4. New boiling point: Tᵦ,new = 100.0 + 1.20 = 101.20°C

Big takeaway: even a “small” temperature shift can imply a pretty concentrated solution when Kᵦ is small (like water).

Frequently Asked Questions

Q: Why does molality use kg of solvent, not liters of solution?

Molality is based on mass of solvent, so it doesn’t change with temperature the way volume-based concentrations can.

Q: Is i always an integer (like 2 for NaCl)?

Ideal i is an integer, but real solutions can have i values that are lower due to ion pairing and non-ideal behavior.

Q: What if my solute is volatile?

The simple boiling point elevation model assumes a nonvolatile solute. Volatile solutes can change vapor pressure differently and need more advanced models.