Problem 49 - Chemistry The Central Science In Si Units

Question

For each statement, indicate whether it is true or false. (a) The solubility of a slightly soluble salt can be expressed in units of moles per liter. (b) The solubility product of a slightly soluble salt is simply the square of the solubility. (c) The solubility of a slightly soluble salt is independent of the presence of a common ion. (d) The solubility product of a slightly soluble salt is independent of the presence of a common ion.

Step-by-Step Solution

Answer

(a) True, (b) False, (c) False, (d) True.

Step 1: Understand solubility expression

The solubility of a slightly soluble salt can indeed be expressed in units of moles per liter, as it refers to the maximum amount that can dissolve in a solvent to form a saturated solution. Thus, statement (a) is true.

Step 2: Analyze solubility product and solubility

The solubility product constant (Ksp) is a specific value derived from the concentrations of the ions in a saturated solution. It is not necessarily the square of the solubility because it can be the product of the concentrations of ions raised to their respective stoichiometric coefficients. Therefore, statement (b) is false.

Step 3: Consider the common ion effect

The solubility of a slightly soluble salt is dependent on the presence of a common ion due to the common ion effect, which reduces solubility. As a result, statement (c) is false.

Step 4: Independence of solubility product from common ion

The solubility product (Ksp) is a fixed value at a given temperature and is not directly affected by the presence of a common ion in the solution. Hence, statement (d) is true.

Key Concepts

Common Ion Effect

The common ion effect is an important concept in chemistry that describes how the solubility of a slightly soluble salt decreases in the presence of a common ion. This occurs because adding an ion that is part of the equilibrium system shifts the balance, leading to decreased solubility. For example, consider a saturated solution of silver chloride ( AgCl ). If we add sodium chloride ( NaCl ), which introduces more chloride ions ( Cl^- ), the equilibrium shifts to allow fewer Ag^+ and Cl^- to dissolve due to this excess. Key points about the common ion effect include:

• The presence of common ions reduces the solubility of the salt.
• This concept is grounded in Le Chatelier's principle, which states that a system at equilibrium tries to counteract changes imposed upon it.
• Understanding this effect is crucial in predicting the solubility of salts in different environments.

Being aware of the common ion effect helps chemists control reactions and predict outcomes, especially in solutions where multiple ionic compounds are present.

Slightly Soluble Salts

Slightly soluble salts refer to those compounds that do not dissolve readily in a solvent, usually water. Unlike highly soluble salts, these create solutions with low ion concentrations, resulting in saturations with much smaller amounts. If you ever notice a precipitate forming when a salt is added to water, chances are you're dealing with a slightly soluble salt. Key characteristics of slightly soluble salts are:

• Their solubility is usually expressed in moles per liter ( mol/L ), indicating the saturation point.
• These salts reach saturation early, meaning that only a few ions go into the solution before equilibrium is achieved.
• They provide an excellent illustration of the dynamic balance between dissolution and precipitation.
• Slightly soluble salts can be affected significantly by changes in the environment such as pH and the presence of other ions.

Understanding the nature of slightly soluble salts helps in various fields from pharmaceuticals to environmental science, where solubility heavily influences processes and reactions.

Solubility Expression

The solubility expression is a way to quantify the dissolution process of compounds in a solvent at a certain temperature. It's usually measured in moles of solute per liter of solvent ( mol/L ). This expression is vital for slightly soluble salts, as it indicates the maximum concentration of ions possible in a solution. When discussing solubility, the solubility product constant ( Ksp ) becomes relevant. This constant is derived from the product of the concentrations of each ion in a saturated solution, raised to the power of their respective coefficients from the balanced dissolution equation. Key aspects about solubility expressions include:

• The solubility expression is critical for determining how much salt can dissolve before reaching saturation.
• It's a straightforward way to compare solubility across different salts.
• Knowledge of the solubility expression aids in predicting the behavior of salts in various chemical reactions and processes.
• Unlike the solubility itself, Ksp remains constant under consistent conditions, providing a reliable measure for equilibrium.

By understanding solubility expressions, one can predict solubility under various conditions and control the conditions for desired chemical reactions.