Problem 99 - Chemistry Matter and Change

Question

Sugar Refining Strontium hydroxide is used in the refining of beet sugar. Only 4.1 g of strontium hydroxide can be dissolved in 1 L of water at 273 K. Given that its solubility is so low, explain how it is possible that strontium hydroxide is considered a strong base.

Step-by-Step Solution

Answer

Strontium hydroxide is a strong base because the dissolved portion fully dissociates into ions, despite its low solubility.

Step 1: Understand the Problem

To understand the problem, recognize that strontium hydroxide is discussed in the context of its solubility and its nature as a base. We need to explain how it is considered a strong base despite its low solubility in water.

Step 2: Define Strong Base

A strong base is a compound that completely dissociates into its ions in an aqueous solution. This means that, even if a strong base is poorly soluble, the amount that dissolves will dissociate completely into hydroxide ions.

Step 3: Solubility vs. Ionization

Although only 4.1 grams of strontium hydroxide dissolves in a liter of water, the dissolved portion completely dissociates into strontium ions (\(Sr^{2+}\)) and hydroxide ions (\(OH^-\)), making it a strong base by providing hydroxide ions.

Step 4: Concentration of Hydroxide Ions

Calculate the molarity of the dissolved strontium hydroxide to determine the availability of hydroxide ions. The formula of strontium hydroxide is (\(Sr(OH)_2\)), so from each mole of strontium hydroxide, you get two moles of hydroxide ions, enhancing its basicity.

Key Concepts

Strontium Hydroxide

Strontium hydroxide is an inorganic compound, known for playing a critical role in various industrial processes such as sugar refining. Its chemical formula is \(Sr(OH)_2\), which indicates that each molecule consists of one strontium atom and two hydroxide ions.

Despite being a compound with relatively low solubility, it is classified as a strong base. This classification is due to the nature of its dissociation in water: it completely separates into its ions, producing hydroxide ions efficiently. When strontium hydroxide dissolves, even in small amounts, the dissolved portion dissociates fully, contributing to the basicity of the solution.

Solubility

Solubility is a key concept in chemistry that refers to the ability of a substance to dissolve in a solvent, forming a homogeneous solution. In the case of strontium hydroxide, solubility is relatively low, as only 4.1 grams can dissolve in one liter of water at 273 K.

Nevertheless, solubility does not directly correlate with a compound's ability to act as a strong base. The distinction here is that the solubility and the strength of a base are separate properties. Solubility is more about how much of the substance can be dissolved, whereas the strength of the base concerns the extent of its dissociation.

Ionization

Ionization is the process by which a compound splits into its constituent ions when dissolved in a solvent. It is a fundamental aspect of understanding why strontium hydroxide is considered a strong base.

Upon dissolving in water, strontium hydroxide fully ionizes, breaking down into strontium ions ( \(Sr^{2+}\) ) and hydroxide ions ( \(OH^-\) ). This complete ionization, even from a small dissolved quantity, is what enhances the basic character of the solution. It effectively increases the concentration of hydroxide ions, which are responsible for the strong basic properties of the solution. This process underscores the importance of ionization in determining the strength of a base.

Aqueous Solution

An aqueous solution is a solution where water acts as the solvent. This type of solution is significant in many chemical reactions, including the dissociation of bases like strontium hydroxide.

In an aqueous solution, water facilitates the complete dissociation of strontium hydroxide into its ions. The molecule interacts with water molecules, which assists in the ionization process. The ions, once separated, remain suspended in the solution. This behavior exemplifies why aqueous solutions can be powerful mediums for chemical reactions, enabling substances like strontium hydroxide to display strong basic properties despite their limited solubility.