Solutions

The One Thing

The behaviour of a solution depends on its composition, the interactions between its particles, and the number of dissolved solute particles. Concentration terms, Raoult’s law, solubility relationships, and colligative-property equations provide quantitative descriptions of this behaviour.

Reactions, Processes and Experiments

Key Terms

• Solution: A homogeneous mixture in which the solute is uniformly distributed throughout the solvent.
• Solute: The component present in the smaller amount or the component that dissolves in a solution.
• Solvent: The component present in the larger amount or the medium in which the solute dissolves.
• Molarity: The number of moles of solute present per litre of solution; it changes with temperature because solution volume can change.
• Molality: The number of moles of solute present per kilogram of solvent; it is independent of temperature because mass does not change with temperature.
• Mole Fraction: The ratio of moles of one component to the total moles of all components in the solution.
• Mass Percentage: The mass of a component expressed as a percentage of the total mass of the solution.
• Parts per Million: A concentration unit used for very dilute solutions, representing parts of solute per million parts of solution.
• Solubility: The maximum amount of a solute that dissolves in a given amount of solvent at a specified temperature and pressure to form a saturated solution.
• Saturated Solution: A solution containing the maximum amount of solute that can dissolve under specified conditions.
• Unsaturated Solution: A solution containing less solute than the maximum amount possible at a given temperature and pressure.
• Ideal Solution: A solution that obeys Raoult's law over the entire composition range, with negligible heat and volume changes on mixing.
• Raoult's Law: For a volatile component, the partial vapour pressure equals its mole fraction multiplied by the vapour pressure of the pure component.
• Non-volatile Solute: A solute with negligible vapour pressure that lowers the vapour pressure of the solvent when dissolved.
• Positive Deviation: A solution shows positive deviation from Raoult's law when solute-solvent attractions are weaker than the attractions among similar molecules.
• Negative Deviation: A solution shows negative deviation from Raoult's law when solute-solvent attractions are stronger than the attractions among similar molecules.
• Colligative Properties: Properties that depend only on the number of solute particles relative to solvent particles, not on their chemical nature.
• Relative Lowering of Vapour Pressure: The fractional decrease in the vapour pressure of a solvent caused by adding a non-volatile solute.
• Elevation of Boiling Point: The increase in the boiling point of a solvent when a non-volatile solute is dissolved in it.
• Depression of Freezing Point: The decrease in the freezing point of a solvent due to the presence of a dissolved solute.
• Osmosis: The movement of solvent molecules through a semipermeable membrane from a dilute solution or pure solvent toward a more concentrated solution.
• Osmotic Pressure: The minimum external pressure required to stop osmosis.
• Van't Hoff Factor: A factor that accounts for the actual number of particles in solution after dissociation or association of solute molecules.
• Azeotrope: A constant-boiling mixture whose liquid and vapour phases have the same composition and cannot be separated completely by fractional distillation.

Easily Confused

• Molarity and molality: Molarity uses moles of solute per litre of solution and varies with temperature; molality uses moles of solute per kilogram of solvent and is temperature-independent.
• Saturated and unsaturated solutions: A saturated solution contains the maximum amount of dissolved solute under specified conditions; an unsaturated solution contains less than this amount.
• Mass percentage and mass by volume percentage: Mass percentage uses mass of solution as the denominator, whereas mass by volume percentage uses volume of solution in millilitres.
• Raoult’s law and Henry’s law: Raoult’s law relates a component’s vapour pressure to its mole fraction in an ideal solution; Henry’s law relates a gas’s partial pressure to its mole fraction in solution.
• Positive and negative deviation: Positive deviation results from weaker unlike-molecule attractions and higher vapour pressure; negative deviation results from stronger unlike-molecule attractions and lower vapour pressure.
• Elevation of boiling point and depression of freezing point: Adding a non-volatile solute raises the boiling point but lowers the freezing point.
• Osmosis and reverse osmosis: Osmosis moves solvent toward the more concentrated solution spontaneously; reverse osmosis uses pressure greater than osmotic pressure to force solvent toward the dilute solution.
• Dissociation and association: Dissociation increases the number of solute particles and generally gives i greater than 1; association decreases the number of particles and generally gives i less than 1.
• Ideal solution and azeotrope: An ideal solution obeys Raoult’s law throughout the composition range, whereas an azeotrope has identical liquid and vapour compositions and cannot be completely separated by fractional distillation.

What Gets Asked

• Calculate concentration using mass percentage, volume percentage, mass by volume percentage, parts per million, molarity, molality, or mole fraction. Marks are lost by using the mass of solution where the formula requires mass of solvent, or by confusing solution volume with solvent mass.
• Apply Raoult’s law to a volatile component or to a binary solution. Marks are lost by omitting the mole fraction or by confusing $p_i$ with the pure-component vapour pressure $p_i^0$.
• Explain why a non-volatile solute causes relative lowering of vapour pressure, elevation of boiling point, and depression of freezing point. Marks are lost by stating that the boiling point decreases or the freezing point increases.
• Use $\Delta T_b=K_{b}m$, $\Delta T_f=K_{f}m$, or their Van’t Hoff-corrected forms. Marks are lost by failing to include $i$ when the solute undergoes association or dissociation.
• Solve osmotic-pressure problems using $\pi =CRT$, $\pi =nRT/V$, $\pi =(w/M)RT/V$, or $\pi =iCRT$. Marks are lost by using Celsius instead of absolute temperature or by ignoring the effect of changed particle number.
• Interpret solubility, Henry’s law, deviations from Raoult’s law, and azeotropes. Marks are lost by reversing the effects of temperature and pressure on gas solubility, or by confusing minimum-boiling and maximum-boiling azeotropes.