Solutions
Domain 6: Solutions
This domain tests concentration, vapour pressure, ideal and non-ideal solutions, and colligative properties, with emphasis on the van't Hoff factor.
Core rules
- Concentration units: molarity (mol/L solution), molality (mol/kg solvent), mole fraction, mass %, ppm. Molality and mole fraction are temperature-independent.
- Raoult's law for ideal solutions: p_total=x_Ap_A°+x_Bp_B°. Positive deviation (weaker A–B forces) gives higher vapour pressure; negative deviation gives lower and can form maximum-boiling azeotropes.
- Colligative properties depend on particle number: relative lowering of vapour pressure, boiling-point elevation ΔTb=iKbm, freezing-point depression ΔTf=iKfm, osmotic pressure π=iCRT.
- van't Hoff factor i is 1 for non-electrolytes; for complete dissociation i equals the number of ions (NaCl -> 2); association gives i<1.
- Henry's law: solubility of a gas is proportional to partial pressure; gases are less soluble at higher temperature.
Worked example
5.85 g NaCl (M=58.5) in enough water to make 1.00 L gives C=0.100 M. At 300 K, assuming complete dissociation, i=2. π=iCRT=2 x 0.100 x 0.0821 x 300=4.93 atm. If NaCl were treated as a non-electrolyte, the answer would be half, showing why i matters.
Traps
- Using molarity in Kb/Kf formulas instead of molality.
- Forgetting i for electrolytes or using i for non-electrolytes.
- Assuming all solutions obey Raoult's law; ethanol–acetone is non-ideal.
- Confusing solubility with concentration.
- Using Celsius in π=iCRT.