SolutionsNEET Chemistry · Class 12 · NCERT Chapter 1

You can expect 3 to 5 questions from Solutions in NEET. Colligative properties (depression in freezing point, elevation in boiling point, osmotic pressure) appear almost every year. van't Hoff factor questions are very frequent in recent NEET papers.

The van't Hoff factor (i) measures how many particles one formula unit of a solute produces in solution. For NaCl it is 2 (Na+ and Cl-), for Na2SO4 it is 3, and for glucose it is 1. You use i to modify colligative property formulas: delta-Tf = i x Kf x m and pi = iMRT. NEET frequently asks you to calculate i from observed molar mass, or to find degree of dissociation from i.

Molarity (M) is moles of solute per litre of solution. Molality (m) is moles of solute per kilogram of solvent. Molality does not change with temperature because it depends on mass rather than volume. Colligative property calculations always use molality.

Raoult's law assumes A-B interactions equal A-A and B-B interactions. If A-B forces are weaker (A-B < A-A, B-B), molecules escape more easily, giving a positive deviation with higher vapour pressure than expected. Example: acetone and ethanol. If A-B forces are stronger, molecules are held more tightly, giving a negative deviation with lower vapour pressure. Example: acetone and chloroform (hydrogen bonding). Azeotropes are constant-boiling mixtures formed at the maximum or minimum of the vapour pressure curve.

Isotonic solutions have equal osmotic pressure. When a cell is placed in a hypertonic solution (higher concentration outside), water leaves the cell and it shrinks (plasmolysis in plant cells). In a hypotonic solution (lower concentration outside), water enters the cell and it swells or bursts (haemolysis in red blood cells). Intravenous fluids must be isotonic with blood plasma (about 0.9% NaCl).

Henry's law states that the partial pressure of a gas above a solution is proportional to its mole fraction in solution: p = KH x chi. Higher KH means the gas is less soluble. It applies to dilute solutions of sparingly soluble gases. Applications include dissolved oxygen in blood (relevant to scuba diving and altitude sickness) and CO2 in carbonated drinks.

Acetic acid has a normal molar mass of 60 g/mol. The observed value of 120 is twice that, so van't Hoff factor i = 60/120 = 0.5. This means molecules are associating: two molecules join through hydrogen bonding to form a cyclic dimer. In benzene (non-polar solvent), acetic acid forms dimers, reducing particle count and making colligative effects smaller than expected.

In normal osmosis, solvent moves from low concentration to high concentration through a semipermeable membrane. Reverse osmosis happens when you apply external pressure greater than the osmotic pressure on the concentrated side, forcing solvent to move in the opposite direction. This is used for water desalination and drinking water purification. The minimum pressure needed equals the osmotic pressure of the solution.