Colligative properties of solutions are properties that depend only on the concentration of solute particles (ions or molecules) dissolved in the solution. Colligative properties include:
Why does solute concentration affect these properties? In liquid solutions, solute particles disrupt the attractive forces between solvent molecules, causing changes to the solvent properties that depend on these attractions.
What’s the effect of electrolytes on colligative properties?
At the same temperature, the vapor pressure of 1M sucrose is greater than the vapor pressure of 1M NaCl. Why is that?
This is because electrolytes such as NaCl contribute more particles per mole than non-electrolytes such as sucrose. And since vapor pressure is a colligative property, its properties will depend only on the concentration of solute particles (ions or molecules) dissolved in the solution. For instance, 1 mol of sucrose will produce 1 mole of sucrose molecules. While 1 mol of NaCl will produce 1 mol of sodium ions (Na+) and 1 mol of chloride ions (Cl–), together that is 2 moles of ions in solution. Similarly, the vapor pressure of 1M NaCl is greater than the vapor pressure of 2M CaCl2. This is because NaCl contributes a total of 2 moles while CaCl2 contributes a total of 3 moles of particles. That is 1mol Ca2+ ions and 2 mol of chloride ions (2Cl– ions).
To consider the dissociation of electrolytes, we will rewrite the boiling point elevation equation as:
ΔTb = Kbmsolute isolute
and the freezing point depression equation as:
ΔTf = = Kfmsolute isolute
where isolute , which is called the van’t Hoff factor, gives the number of particles per formula unit of solute.
For non-electrolytes, isolute is equal to 1, mainly because they do not dissociate in solution
For electrolytes, isolute is equal to the number of ions per formula unit of the ionic compound in dilute solutions. For example, isolute for NaCl is 2, and isolute for CaCl2 is 3.
However, note that the actual isolute value must be experimentally determined.