Q.The solubility of N₂ gas in water at 25 ⁰C and 1 bar is 6.85 × 10⁻⁴ mol L⁻¹. Calculate

Solubility is the amount of solute a saturated solution holds per unit volume, at a specific temperature -- the solute's maximum achievable concentration in that solvent at that temperature, usually expressed in mol L-1. A solution reaches saturation through a dynamic equilibrium between dissolution and crystallisation; adding solute beyond this point (a supersaturated solution) is unstable and will precipitate the excess if disturbed. Three factors govern solubility: (1) the chemical nature of solute and solvent -- 'like dissolves like', since similar intermolecular forces favour mutual solubility; (2) temperature, whose effect depends on whether that solute's dissolution is endothermic (solubility rises with temperature, by Le Chatelier's principle, e.g. KCl) or exothermic (solubility often falls, e.g. Na2SO4) -- though this is not a strict universal rule; (3) pressure, which has no effect on solids/liquids (incompressible) but strongly governs GAS solubility via Henry's law, $S=K_HP$ -- the solubility of a gas is directly proportional to its partial pressure above the solution, with $K_H$ (Henry's law constant) as the proportionality constant. Gas solubility in water generally decreases with rising temperature (dissolving a gas is itself exothermic); NH3 and CO2 are noted exceptions to plain Henry's law because they react chemically with water, and O2 dissolves far more in blood than in water alone because it chemically binds haemoglobin.