What is the difference between molality and molarity?

Molality is moles of solute per kilogram of solvent, independent of temperature. Molarity is moles of solute per liter of solution and varies with temperature.

All calculators

Colligative Properties Calculator

Q: What values of i should I use?

Use i≈1 for nonelectrolytes (like sugar), i≈2 for NaCl, i≈3 for CaCl2, etc. Real values may deviate from theory.

Q: Where do Kb and Kf come from?

Kb and Kf are solvent-specific constants provided in tables or textbooks. For water, Kb=0.512 °C·kg/mol and Kf=1.86 °C·kg/mol.

Compute boiling point elevation (ΔT_b) and freezing point depression (ΔT_f) using the colligative relations ΔT_b = iK_bm and ΔT_f = iK_fm. Choose Simple to enter molality directly, or Advanced to have molality calculated from masses.

Background

Colligative properties depend only on the number of solute particles, not their identity. The van’t Hoff factor i accounts for dissociation/association (e.g., NaCl → i≈2). K_b and K_f are solvent-specific constants (°C·kg·mol⁻¹).

Choose mode and enter values:

Mode:

Simple (enter molality m)

Advanced (compute m from masses)

Compute:

Boiling point elevation (ΔT_b & new BP)

Freezing point depression (ΔT_f & new FP)

Solvent constants & normal points:

K_b (°C·kg/mol):

K_f (°C·kg/mol):

Normal BP (°C):

Normal FP (°C):

Quick solvents:

Molality m (mol/kg solvent):

van’t Hoff factor i:

Quick examples:

Result:

No results yet. Pick a mode, enter values, and calculate.

How to use this calculator

1) Choose Simple (enter molality) or Advanced (compute molality from masses).
2) Enter solvent constants (or click a Quick solvent).
3) Enter i (van’t Hoff factor) and either m or the masses to compute m.
4) Click Calculate to see ΔT_b, ΔT_f and new boiling/freezing points.

Formulas

ΔT_b = iK_bm, New BP = BP_normal + ΔT_b
ΔT_f = iK_fm, New FP = FP_normal − ΔT_f
m (molality) = (moles of solute) / (kg of solvent)

Example Problems & Step-by-Step Solutions

Example 1 (Simple)

Water with m = 0.200 mol/kg of a non-electrolyte (i=1.0). K_b=0.512, K_f=1.86.
ΔT_b=iK_bm=1.0×0.512×0.200=0.1024 °C → BP≈100.10 °C.
ΔT_f=iK_fm=1.0×1.86×0.200=0.372 °C → FP≈−0.372 °C.

Example 2 (Advanced)

5.00 g NaCl (M=58.44) dissolved in 100.0 g water. i≈2.0, K_f=1.86.
n = 5.00/58.44 = 0.0856 mol; kg solvent = 0.1000 kg; m = 0.856 mol/kg.
ΔT_f = iK_fm = 2.0×1.86×0.856 ≈ 3.19 °C → FP≈ −3.19 °C.