Question

State Raoult's law for a solution containing volatile components. Write any two characteristics of a solution which obeys Raoult's law at all concentrations.

Hint:

Ideal solutions perfectly obey Raoult's law without any volume or enthalpy change upon mixing.

Answer 1

Step 1: State Raoult's law.
Raoult's law states that for a solution of volatile liquids, the partial vapour pressure of each component is directly proportional to its mole fraction in the solution at constant temperature: $p_A = p_A^\circ \cdot x_A$ and $p_B = p_B^\circ \cdot x_B$.
Step 2: Express total vapour pressure.
The total vapour pressure of the solution is the sum of the partial pressures of all volatile components: $p_{\text{total}} = p_A^\circ x_A + p_B^\circ x_B$.
Step 3: Define ideal solution.
A solution that obeys Raoult's law at all concentrations is called an ideal solution. This occurs when solute-solvent (A-B) intermolecular forces are identical to pure-component (A-A and B-B) forces.
Step 4: Two characteristics of an ideal solution.
(1) Enthalpy of mixing is zero: $\Delta H_{\text{mix}} = 0$ (no heat is absorbed or released on mixing). (2) Volume of mixing is zero: $\Delta V_{\text{mix}} = 0$ (total volume equals sum of individual volumes).