Chemistry · Solutions · NEET
It is just a counting number. i = (number of particles you have AFTER the solute dissolves) divided by (number of particles you put in BEFORE dissolving). If nothing changes, i = 1. If one unit breaks into 2 ions, i = 2. If two units join into one, i = 0.5. That is all it means.
For a strong electrolyte that fully splits, just count the ions one formula unit makes. NaCl → Na+ + Cl- = 2 ions, so i = 2. BaCl2 → Ba2+ + 2Cl- = 3 ions, so i = 3. Ba(OH)2 → Ba2+ + 2OH- = 3 ions, so i = 3. K4[Fe(CN)6] → 4K+ + 1 complex ion = 5 ions, so i = 5. This works because NEET usually says 'strong electrolyte' meaning complete dissociation.
More than 1 (i > 1) means DISSOCIATION: one solute unit breaks into several ions, so there are more particles than you started with. Less than 1 (i < 1) means ASSOCIATION: several solute units clump into one bigger particle, so there are fewer particles. Example of i<1: benzoic acid or acetic acid in benzene forms dimers (two molecules join as one), giving i ≈ 0.5.
Colligative properties (like freezing point drop) depend on the NUMBER of particles. If a solute splits or joins, the particle count is not what you expect, so the molar mass you calculate from experiment comes out wrong — that wrong value is the 'abnormal' (observed) molar mass. The link is simple: i = normal (true) molar mass / abnormal (observed) molar mass. If it dissociates, observed molar mass is smaller than true, so i > 1.
For a solute that gives n particles: i = 1 + (n - 1)α, where α is the fraction that dissociated. For association where m molecules join: i = 1 + (1/m - 1)α, and here i < 1. Quick check: if α = 1 (fully dissociated) then i = n, which matches ion counting.
You simply multiply the normal formula by i. Freezing point: ΔTf = i·Kf·m. Boiling point: ΔTb = i·Kb·m. Osmotic pressure: π = i·C·R·T. Relative lowering of vapour pressure also uses i. So the solution with the biggest i × molality product shows the biggest effect.
The van't Hoff factor (i) for a dilute aqueous solution of the strong electrolyte barium hydroxide is
Which amongst the following aqueous solutions of electrolytes will have minimum elevation in boiling point?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Not always. For strong electrolytes that fully split, i is close to a whole number (2, 3, 5). But for weak electrolytes that only partly dissociate, i is a decimal between 1 and n. For association it can be a fraction below 1, like 0.5 for a dimer.
i = 1. Glucose, urea and sucrose are non-electrolytes — they do not split into ions and do not join together, so the particle count stays the same. In NEET problems, if a solute is called a 'non-electrolyte', use i = 1.
i > 1 means more particles, so the freezing point DROPS more (bigger depression, ΔTf = i·Kf·m). So an electrolyte freezes at a lower temperature than a non-electrolyte of the same molality.
Because the experiment measures particle count, not molecule count. An electrolyte makes extra particles by splitting, so the measured (observed) molar mass comes out smaller than the true value. i = true molar mass / observed molar mass corrects this.
Acetic acid forms dimers (two molecules join as one) in benzene, so the particle count roughly halves and i ≈ 0.5. This is association, giving i < 1 and a larger observed molar mass than normal.
