How much does salt lower the freezing point of water?

For 1.00 mol of NaCl per kg of water: ΔT = 2 × 1.86 × 1.00 = 3.72 °C, so the solution freezes at about −3.7 °C.

Why is molality used instead of molarity?

Molality is based on mass of solvent, so it doesn't change with temperature — important when the temperature itself is what you're measuring.

How are colligative properties used to find molar mass?

Measure ΔT for a known mass of solute in a known mass of solvent, solve for molality, then divide the grams of solute by the moles.

Colligative Properties Calculator · PeriodicElementTable.com

ΔT = i × K × m — fill in every field except the one you want; the empty field is solved for.

Solvent constant (°C·kg/mol):

Change in boiling or freezing point (ΔT)

van 't Hoff factor (i)1 for sugar, about 2 for NaCl, about 3 for CaCl₂

Solvent constant (K_b or K_f) (K)

°C·kg/mol

Molality (m)

mol/kg

Change in boiling or freezing point (ΔT) = 1.86 °C

Rearrange: ΔT = i × K × m
Substitute: (2) × (1.86 °C·kg/mol) × (0.5 mol/kg) = 1.86 K

How it works

For boiling and freezing points, ΔT = i × K × m, where m is molality (mol of solute per kg of solvent) and K is the solvent's ebullioscopic (K_b) or cryoscopic (K_f) constant. The result is the size of the change: add it to the pure solvent's boiling point, or subtract it from the freezing point.

Osmotic pressure uses Π = iMRT with molarity and R = 0.082057 L·atm/(mol·K). The van 't Hoff factor i is the number of particles per formula unit: 1 for sugar, close to 2 for NaCl and 3 for CaCl₂ in dilute solution (slightly less in practice because ions pair up).

Frequently asked questions

How much does salt lower the freezing point of water?: For 1.00 mol of NaCl per kg of water: ΔT = 2 × 1.86 × 1.00 = 3.72 °C, so the solution freezes at about −3.7 °C.
Why is molality used instead of molarity?: Molality is based on mass of solvent, so it doesn't change with temperature — important when the temperature itself is what you're measuring.
How are colligative properties used to find molar mass?: Measure ΔT for a known mass of solute in a known mass of solvent, solve for molality, then divide the grams of solute by the moles.

How it works

Frequently asked questions

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