Solutions

The One Thing

The properties of a solution depend both on its composition and, for colligative properties, on the effective number of dissolved solute particles. Vapour-pressure, boiling-point, freezing-point and osmotic-pressure measurements can therefore be used to determine concentration and molar mass, provided association or dissociation is accounted for using the van’t Hoff factor.

Reactions, Processes and Experiments

Key Terms

• Solution: A homogeneous mixture containing one or more solutes dissolved in a solvent.
• Solute: The component present in a smaller amount that dissolves in the solvent.
• Solvent: The component present in a larger amount that dissolves the solute.
• Molarity: The number of moles of solute present per litre of solution; it changes with temperature because solution volume changes.
• Molality: The number of moles of solute present per kilogram of solvent; it is independent of 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 solute divided by the mass of solution, multiplied by 100.
• Parts per million: The concentration expressed as parts of solute per million parts of solution, commonly used for very dilute solutions.
• Vapour pressure: The pressure exerted by vapour in equilibrium with its liquid at a given temperature.
• Raoult's law: For an ideal solution, the partial vapour pressure of each component equals its mole fraction multiplied by the vapour pressure of the pure component.
• Ideal solution: A solution that obeys Raoult's law over the entire composition range, with nearly unchanged volume and heat on mixing.
• Non-ideal solution: A solution that shows positive or negative deviation from Raoult's law because solute-solvent interactions differ from solute-solute or solvent-solvent interactions.
• Colligative properties: Properties that depend on the number of solute particles relative to solvent particles rather than on the identity of the solute.
• Relative lowering of vapour pressure: The lowering of solvent vapour pressure divided by the vapour pressure of pure solvent; for a dilute solution of a non-volatile solute, it equals the solute mole fraction.
• Elevation of boiling point: The increase in boiling point of a solution compared with the pure solvent due to reduced vapour pressure.
• Depression of freezing point: The decrease in freezing point of a solution compared with the pure solvent.
• Osmosis: Movement of solvent 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.
• Semipermeable membrane: A membrane that permits solvent molecules to pass through but prevents solute particles from passing.
• Van’t Hoff factor: The ratio of the observed colligative property to the theoretically calculated value, accounting for association or dissociation of solute particles.
• Abnormal molar mass: A molar mass calculated from colligative properties that differs from the true value because solute particles associate or dissociate.

Easily Confused

• Molarity and molality: Molarity uses litres of solution and changes with temperature; molality uses kilograms of solvent and is independent of temperature.
• Solute and solvent: The solute is the component in the smaller amount that dissolves; the solvent is the component in the larger amount that dissolves the solute.
• Ideal and non-ideal solutions: An ideal solution obeys Raoult's law throughout the composition range; a non-ideal solution shows positive or negative deviation.
• Boiling-point elevation and freezing-point depression: A non-volatile solute raises the boiling point but lowers the freezing point.
• Osmosis and osmotic pressure: Osmosis is the movement of solvent through a semipermeable membrane; osmotic pressure is the minimum external pressure required to stop that movement.
• Association and dissociation: Association decreases the number of particles and generally gives i < 1; dissociation increases the number of particles and generally gives i > 1.
• Homogeneous solution and suspension or colloid: A solution has uniform composition throughout, whereas a suspension or many colloids are not true homogeneous solutions.
• Theoretical and abnormal molar mass: Abnormal molar mass results when association or dissociation changes the effective number of solute particles.

What Gets Asked

• Concentration calculations: Questions may require n = given mass / molar mass, molarity, molality, mole fraction, mass percentage, or parts per million. Marks are commonly lost by using the volume of solution in a molality calculation or the mass of solvent in a molarity calculation.
• Temperature dependence: Questions may ask which concentration unit changes with temperature. The essential distinction is that molarity depends on solution volume, whereas molality depends on solvent mass.
• Raoult's-law and vapour-pressure problems: Questions may require p_i = x_i p_i^0 or the relative-lowering expressions. Marks are lost by confusing the vapour pressure of the pure solvent, p^0, with the vapour pressure of the solution, p.
• Boiling-point and freezing-point calculations: Questions may require Delta T_b = K_b m or Delta T_f = K_f m, including i where appropriate. The relevant temperature changes are T_b - T_b^0 for boiling and T_f^0 - T_f for freezing.
• Osmotic-pressure and molar-mass determination: Questions may use pi = C R T, pi = i C R T = i n R T / V, or M_2 = w_2 R T / (pi V). Marks are lost by omitting the van’t Hoff factor when the solute associates or dissociates.
• Association and dissociation: Questions may ask for the effect on particle number, van’t Hoff factor, or abnormal molar mass. Association requires i < 1, while dissociation requires i > 1; complete dissociation of NaCl gives approximately two ions and i close to 2 in a sufficiently dilute solution.