Colligative Properties Calculator

Colligative Properties Calculator

°C/m
mol/kg

What are Colligative Properties?

Colligative properties are properties of solutions that depend solely on the number of solute particles dissolved in a solvent, not on the identity of the solute particles themselves. In dilute ideal solutions, these properties are directly proportional to the concentration of solute particles.

The main colligative properties are:

  • Vapor Pressure Lowering: The vapor pressure of a solvent is lowered when a non-volatile solute is added.
  • Boiling Point Elevation (ΔTb): The boiling point of a solvent increases when a solute is dissolved in it.
  • Freezing Point Depression (ΔTf): The freezing point of a solvent decreases when a solute is dissolved in it.
  • Osmotic Pressure (Π): The pressure required to prevent the flow of solvent across a semipermeable membrane from a region of pure solvent to a solution.

This calculator focuses on boiling point elevation, freezing point depression, and osmotic pressure.

Frequently Asked Questions

What is the Van 't Hoff factor (i)?

The Van 't Hoff factor (i) represents the number of distinct ions or particles produced when one formula unit of a solute dissolves.
• For non-electrolytes (substances that don't dissociate into ions, like sugar or ethanol), i = 1.
• For strong electrolytes (substances that fully dissociate, like NaCl or MgCl₂), i is equal to the total number of ions produced per formula unit (e.g., i = 2 for NaCl → Na⁺ + Cl⁻; i = 3 for MgCl₂ → Mg²⁺ + 2 Cl⁻).
• For weak electrolytes, i is between 1 and the theoretical maximum number of ions, depending on the degree of dissociation. For simplicity, calculations often use the ideal maximum value.

Why use molality (m) for boiling/freezing point but molarity (M) for osmotic pressure?

Boiling point elevation and freezing point depression are directly related to the ratio of solute particles to solvent particles. Molality (moles of solute per kg of solvent) is temperature-independent because it's based on mass. Molarity (moles of solute per L of solution), however, can change slightly with temperature because the volume of the solution can expand or contract. Since boiling and freezing points involve temperature changes, molality provides a more stable concentration measure. Osmotic pressure calculations are typically done assuming a constant temperature, where molarity is more convenient and aligns with the ideal gas law constant (R) units.

What are Kb and Kf?

Kb (Ebullioscopic Constant) and Kf (Cryoscopic Constant) are properties specific to the solvent. They quantify how much the boiling point or freezing point changes for every 1 molal concentration of solute particles. For example, water has Kb = 0.512 °C/m and Kf = 1.86 °C/m. These values must be looked up or provided for the specific solvent being used.