Chemistry · Ch 9 — Solutions
Summary
Summary
- A solution is a homogeneous mixture of two or more chemically non-reacting substances, mixed uniformly; the component present in the larger proportion is the solvent, the lesser component is the solute.
- The concentration units covered are formality, molality, normality, molarity, mole fraction, %w/w, %w/v, %v/v, and, for very small solute quantities, ppm.
- Standard solutions are prepared accurately and then diluted to whatever concentration is needed (working standards), which overcomes weighing error and keeps preparation efficient and consistent.
- Solubility is the amount of solute (in grams) present in a saturated solution per 100 g of solvent; it depends on the nature of the solute and solvent, and on temperature.
- The solubility of a gas in a liquid depends on the nature of the gas and liquid, the system's temperature, and the gas's pressure -- the pressure dependence is governed by Henry's law: a gas's solubility in a liquid, at a given temperature, is directly proportional to its partial pressure.
- Vapour pressure of a liquid is the pressure exerted by its vapour when in dynamic equilibrium with the liquid, in a closed container. Raoult's law states that the vapour pressure of a solution containing a nonvolatile solute is directly proportional to the mole fraction of the solvent (), with the pure solvent's own vapour pressure as the proportionality constant.
- A solution shows positive deviation from Raoult's law if its vapour pressure is higher than predicted (e.g. ethyl alcohol and cyclohexane), and negative deviation if its vapour pressure is lower than predicted (e.g. acetone and chloroform).
- Colligative properties are properties of a solution that depend only on the number of solute particles present, not on their chemical nature. They are:
- (a) Relative lowering of vapour pressure -- adding a nonvolatile solute lowers the solution's vapour pressure below the pure solvent's; by Raoult's law, the relative lowering equals the mole fraction of the solute.
- (b) Elevation of boiling point () -- the solution boils at a higher temperature than the pure solvent, because its (lowered) vapour pressure needs a higher temperature to climb back up to atmospheric pressure.
- (c) Depression of freezing point -- the solution's freezing point is lower than the solvent's, because the temperature at which the solid and liquid have equal vapour pressure is lower for the solution. …