MCAT General Chemistry · Lesson 9
Solutions
2 min read4 sectionsUpdated
4 sections
9.1 Nature of Solutions
Solutions are homogeneous mixtures of solute in solvent. Solvation involves interactions between solute and solvent.
- Solution & Solvation
- Solute + Solvent → homogeneous mixture.
- Dissolution: solute-solvent interactions.
- Hydration: water is solvent.
- Exothermic if new interactions stronger → favored at low T.
- Endothermic if new interactions weaker → favored at high T.
- Ideal solutions: ∆H ≈ 0.
- Entropy generally increases upon dissolution.
- Solubility
- Maximum solute that can dissolve at given T and P.
- Saturated: solution contains maximum solute.
- Sparingly soluble salts: minimal solubility, increases with heating.
- Aqueous Solubility Rules
- All salts of ammonium & alkali metals soluble.
- All nitrates & acetates soluble.
- Halides soluble except F⁻ and Ag⁺, Pb²⁺, Hg₂²⁺.
- Sulfates soluble except Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺.
- Metal oxides generally insoluble.
- OH⁻ generally insoluble; exceptions Ca²⁺, Sr²⁺, Ba²⁺.
- Carbonates, phosphates, sulfides, sulfites insoluble.
- Complex Ions
- Central cation + ligands (coordinate covalent bonds).
- Chelation: single ligand binds multiple sites.
- Important for cofactors, coenzymes, toxic metal sequestration.
9.2 Concentration
Quantifying solute in solution using different units.
- Units
- % composition by mass: mass solute / mass solution × 100.
- Mole fraction: moles A / total moles.
- Molarity (M): moles solute / L solution.
- Molality (m): moles solute / kg solvent (≈ M at 25°C for water).
- Normality (N): equivalents of interest / L solution.
- Dilution
9.3 Solution Equilibria
Saturation, solubility product, and effects of complex ions and common ions.
- Saturation
- Equilibrium: rate of dissolution = rate of precipitation.
- Solubility Product
- Ion product (IP): analogous to Q.
- → unsaturated.
- → saturated.
- → supersaturated.
- Complex Ions
- Formation constant Kf >> Ksp.
- Large Kf drives dissolution forward.
- Common Ion Effect
- Pre-existing ions reduce solubility of salt.
- Does not change Ksp.
9.4 Colligative Properties
Physical properties depend on number of solute particles, not their identity.
- Raoult's Law
- Vapor pressure depression:
- Boiling Point Elevation
- Freezing Point Depression
- Salt roads in winter: lowers freezing point.
- Osmotic Pressure
- Water moves toward higher solute concentration.
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