Question

Why does the amount of excess solid solute present in a solution not affect the amount of solute that ultimately dissolves in a given amount of solvent?

Short answer

The amount of solute that ultimately dissolves in a given amount of solvent is not affected by the presence of excess solid solute due to the solubility equilibrium, which maintains a constant solute concentration in the solution. Additionally, the solubility product constant, K_sp, only depends on the concentrations of dissolved ions and remains constant, irrespective of the excess solid solute. Consequently, even when excess solid solute is present, the rate of dissolution and precipitation remain equal under the solubility equilibrium, and thus, the solute dissolution process remains unaffected.

Step-by-step solution

Understanding Solubility Equilibrium

Solubility equilibrium refers to the balance between the rate of dissolution of a solute in a solvent and the rate of precipitation (formation of solid) from the solution. When a solute dissolves in a solvent, its concentration in the solution increases until it reaches a maximum value called the solubility limit. At this point, the rate of dissolution equals the rate of precipitation, and the solute's concentration in the solution remains constant. This dynamic balance is called solubility equilibrium.

Presence of Excess Solute

When a solution is saturated, it means that the maximum amount of solute has dissolved in the solvent per the given conditions (e.g., temperature). At saturation, the concentration of solute in the solution is equal to its solubility limit. If there is excess solid solute present in the solution (beyond the equilibrium concentration), it will not dissolve, as the solution cannot accommodate more of the solute.

Role of Equilibrium Constant

In a saturated solution, the equilibrium constant, K_sp (solubility product constant), represents the interaction between the dissolved solute ions and the solid solute. K_sp is calculated using the concentrations of the dissolved ions, raised to the power of their stoichiometric coefficients. The value of K_sp remains constant, irrespective of the presence of excess solid solute, as it only depends on the concentrations of the dissolved ions.

Reasons for No Effect on Solute Dissolution

The amount of solute that ultimately dissolves in a given amount of solvent is not affected by the presence of excess solid solute due to the following reasons: 1. Solubility equilibrium: The dynamic equilibrium between the dissolved solute and excess solid solute maintains constant solute concentration in the solution. 2. Equilibrium constant: K_sp is not affected by the presence of excess solid solute, as it depends only on the concentrations of the dissolved ions. 3. Mass action effect: Even if there is an excess of solid solute, the solution does not get affected since the rate of dissolution and precipitation remain equal under the solubility equilibrium. In conclusion, the excess solid solute does not influence the solute dissolution process because the solubility equilibrium maintains a constant concentration of solute in the solution regardless of the presence of additional solid solute.