Colligative properties

Colligative properties definition

Properties that depend only on the number of solute particles in solution, not their chemical identity.

Four colligative properties

Lowering of vapor pressure; Boiling point elevation; Freezing point depression; Osmotic pressure.

Additive properties

Properties depending on total number of atoms in a molecule (e.g., molecular weight).

Constitutive properties

Properties depending on structural arrangement of atoms (e.g., optical rotation, refractive index).

Raoult’s Law formula

Ptotal = P°solvent × Xsolvent

Boiling point elevation formula

ΔTb = Kb × m

Freezing point depression formula

ΔTf = Kf × m

Osmotic pressure formula

π = mRT (or MRT depending on units)

Molality definition

Moles of solute per kilogram of solvent.

Effect of nonvolatile solute on vapor pressure

Decreases vapor pressure by blocking solvent molecules from escaping surface.

Why solute increases boiling point

More heat is needed for vapor pressure to reach atmospheric pressure.

Why solute decreases freezing point

Solute interferes with crystal lattice formation, requiring lower temperature to freeze.

Define osmolarity

Number of dissolved particles per liter of solution.

Define osmolality

Number of dissolved particles per kilogram of solvent.

Physiologic osmolarity

Approximately 291–300 mOsm/L.

Isotonic solution definition

Same osmotic pressure as RBCs → no change in cell volume.

Hypertonic solution definition

Higher osmotic pressure → water leaves RBCs → shrink (crenation).

Hypotonic solution definition

Lower osmotic pressure → water enters RBCs → swelling and hemolysis.

Isoosmotic vs isotonic difference

Isoosmotic may not be isotonic if solute penetrates cells (e.g., urea → hemolysis).

Van’t Hoff factor (i)

Number of particles an electrolyte dissociates into; affects colligative strength.

Osmosis definition

Movement of solvent toward higher solute concentration across a semipermeable membrane.

Osmotic pressure physiological importance

Maintains fluid balance; affects ADH release and thirst.

Oros (osmotic pump) advantage

Provides controlled, constant drug release over time.

Colligative use in pharmacy

Determine MW or adjust tonicity of solutions.

Identify solution with osmolarity <291 mOsm/L*

Hypotonic.*

Identify solution with osmolarity >300 mOsm/L

Hypertonic.

Identify solution with osmolarity ~291-300 mOsm/L

Isotonic.

How do you classify solutions by osmolarity for IV use?

• <291 mOsm/L = Hypotonic → water enters cells → hemolysis risk (not for IV injection)

• 291-300 mOsm/L = Isotonic → safe for IV, no change in cell volume

• >300 mOsm/L = Hypertonic → water leaves cells → cell shrinkage (use with caution)

Clinical rule: Hypotonic IV solutions

CANNOT be injected into blood → cause hemolysis.

Clinical rule: Hypertonic IV solutions

Must be infused slowly → can irritate veins and shift fluid.

Osmolarity of purified water

0 mOsm/L → Hypotonic

D5W (5% dextrose water) tonicity

tonicity 252 mOsm/L

Hypotonic after metabolism of glucose → hemolysis risk if rapid infusion.

0.9% Sodium Chloride (Normal Saline) tonicity 308 mOsm/L

Isotonic → safe for IV administration.

0.45% Sodium Chloride (Half NS) tonicity

154 mOsm/L

Hypotonic → water enters cells.

3% Sodium Chloride tonicity

1026 mOsm/L

Hypertonic → water leaves cells.

Lactated Ringer’s solution tonicity

Isotonic → common for fluid replacement.

Freezing point depression of isotonic solution

−0.52°C.

Which colligative property is used to assess isotonicity?

Freezing point depression.

Main risk of rapid hypotonic infusion

Hemolysis and possible cerebral edema.

Main risk of rapid hypertonic infusion

Vein irritation, dehydration of RBCs → crenation.