What is the van't Hoff factor? How does it explain abnormal molar masses? Derive the relation between i and the degree of dissociation of an electrolyte.

1. 1 · Define. i = observed colligative property ÷ calculated colligative property = normal molar mass ÷ abnormal molar mass.
2. 2 · Explain dissociation. Electrolytes such as KCl split into ions in water, so there are more particles: i > 1, and the molar mass found is lower than the true value.
3. 3 · Explain association. Ethanoic acid forms dimers in benzene through hydrogen bonding, so there are fewer particles: i < 1, and the molar mass found is higher (about 120 g mol⁻¹).
4. 4 · Derive α.
   • One formula unit gives n ions; α is the fraction dissociated
   • Particles from 1 mol = (1 − α) + nα = 1 + (n − 1)α
   • i = 1 + (n − 1)α, so α = (i − 1) ÷ (n − 1)
5. 5 · Write the modified equations. ΔTb = i Kb m, ΔTf = i Kf m, π = i CRT.
6. 6 · Link to real life. ORS and saline drips are made by counting particles: each NaCl gives two.
7. 7 · Close in one line. So i turns the expected number of particles into the real number.

Scoring tip: Write the particle-count line, (1 − α) + nα, clearly. That single line usually carries a mark.