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 Incoming links: Molality

A colligative property is a solution property whose dilute ideal limit depends on the number of dissolved particles relative to the solvent, rather than on their detailed identity. Examples include freezing-point depression, boiling-point elevation and osmotic pressure. Dissociation matters because one formula unit of dissolved sodium chloride can supply two ions.^[ref]

For dilute aqueous chemical solutions, the freezing-point change is approximately

Δ T_{f} = i K_{f} m

, where

m

K_{f}

depends on the solvent, and

i

is the effective particle-number factor. Taking

K_{f} = 1.86 K kg mo l^{- 1}

0.10 mol k g^{- 1}

glucose gives about

0.186 K

depression, while ideally dissociated sodium chloride at the same molality gives about

0.372 K

. Interactions make real concentrated solutions depart from this ideal counting rule.^[ref]

Ionic product of water Created 2026-10-20 Updated 2026-11-05

2 H_{2} O ⇌ H_{3} O^{+} + OH^{-} .

For a dilute water-based chemical solution whose water activity is approximately one,

K_{w} ≃ a_{H_{3} O^{+}} a_{O H^{-}} .

The ionic thermodynamic activities are dimensionless. In a dilute molal convention,

a_{i} ≃ b_{i} / b^{\circ}

, where

b_{i}

b^{\circ} = 1 mol k g^{- 1}

. The full equilibrium constant for the reaction includes water activity squared in the denominator; concentrated solutions need that solvent contribution as well.

Near ambient pressure at

25^{\circ} C

K_{w}

is approximately

1 0^{- 14}

. Equal ionic activities in neutral dilute water are therefore approximately

1 0^{- 7}

, giving pH near seven. The value changes with temperature and pressure, so neutrality is not universally the same as pH seven. ^[ref]

Molality 2026-11-05

b = n_{solute} / m_{solvent}

. Its SI unit is

mol k g^{- 1}

. Dissolving

0.10 mol

of solute in

0.50 kg

of solvent produces

b = 0.20 mol k g^{- 1}