Problem 29 - Chemistry The Molecular Science

Question

Predict whether each compound is soluble in water. Indicate which ions are present in solution for the watersoluble compounds. (a) Potassium monohydrogen phosphate (b) Sodium hypochlorite (c) Magnesium chloride (d) Calcium hydroxide (e) Aluminum bromide

Step-by-Step Solution

Answer

(a) Soluble: K\(^+\), HPO\(_4\)^{2-}; (b) Soluble: Na\(^+\), ClO\(^-\); (c) Soluble: Mg\(^{2+}\), Cl\(^-\); (d) Slightly soluble; (e) Soluble: Al\(^{3+}\), Br\(^-\).

Step 1: Understand Solubility Rules

We need to understand the general solubility rules in order to predict whether each compound will dissolve in water. Some key rules are as follows: 1. Compounds containing alkali metal ions (such as potassium (K\(^+\)) and sodium (Na\(^+\))) are usually soluble.2. Chlorides, bromides, and iodides are generally soluble, except for those of silver, lead, and mercury.3. Most phosphates are insoluble, except when paired with alkali metals or ammonium.

Step 2: Assess Potassium Monohydrogen Phosphate

Potassium monohydrogen phosphate (K\(_2\)HPO\(_4\)) contains potassium ions, which fall under the category of alkali metals. According to rule 1, potassium salts are soluble in water. Thus, K\(_2\)HPO\(_4\) is water-soluble.The ions present in the solution are: K\(^+\) and HPO\(_4\)^{2-}.

Step 3: Assess Sodium Hypochlorite

Sodium hypochlorite (NaClO) contains sodium, which is an alkali metal ion. As per rule 1, sodium compounds are soluble in water.Thus, NaClO is water-soluble.The ions present in the solution are: Na\(^+\) and ClO\(^-\).

Step 4: Assess Magnesium Chloride

Magnesium chloride (MgCl\(_2\)) contains chloride ions, and rule 2 states that chlorides are generally soluble unless paired with Ag, Pb, or Hg. Thus, MgCl\(_2\) is water-soluble.The ions present in the solution are: Mg\(^{2+}\) and Cl\(^-\).

Step 5: Assess Calcium Hydroxide

Calcium hydroxide (Ca(OH)\(_2\)) is only slightly soluble in water. While some of the solute may dissolve, it is generally considered to have limited solubility. Thus, Ca(OH)\(_2\) is only slightly soluble in water.

Step 6: Assess Aluminum Bromide

Aluminum bromide (AlBr\(_3\)) contains bromide ions. Rule 2 states that bromides are generally soluble except in the presence of silver, lead, and mercury.Thus, AlBr\(_3\) is water-soluble.The ions present in the solution are: Al\(^{3+}\) and Br\(^-\).

Key Concepts

Understanding Water-Soluble Compounds

When we talk about solubility in chemistry, we are discussing how certain substances dissolve in solvents, like water. Water-soluble compounds are those that readily dissolve to form a homogeneous mixture in water. This property depends on the interaction between water molecules and the ions that make up the compound.
A compound is considered water-soluble if its dissolution leads to the separation of its ions in the water.
Solubility is governed by some general rules:

• Compounds containing alkali metal ions (like sodium and potassium) are typically soluble.
• Compounds with ammonium ions (NH extsubscript{4} extsuperscript{+}) are also usually soluble.
• Most nitrates (NO extsubscript{3} extsuperscript{-}) and acetates (CH extsubscript{3}COO extsuperscript{-}) are soluble.

Identifying these common traits in compounds helps in predicting their solubility behavior. Understanding which compounds are water-soluble is essential in fields such as chemistry, biology, and environmental science.

The Role of Alkali Metals in Solubility

Alkali metals are a group of elements found in group 1 of the periodic table, including elements like lithium (Li), sodium (Na), and potassium (K). These metals have a single electron in their outermost shell, making them highly reactive.
In the context of solubility, compounds containing alkali metal ions are very easy to dissolve in water.
Why is this the case?

• Alkali metal ions are relatively small with a single positive charge, which allows them to interact strongly with water molecules.
• This interaction helps to effectively separate the ions, leading to their dissolution in water.

Examples include sodium chloride (NaCl) and potassium nitrate (KNO extsubscript{3}), both of which are highly soluble in water. Understanding these properties can simplify the prediction of whether a compound with alkali metals will be water-soluble.

Chlorides and Bromides Solubility Explained

Chlorides and bromides are compounds formed by the combination of chlorine or bromine with a metal or another positive ion. These compounds, in general, are known for their solubility in water, thanks to their ability to dissociate completely in solution.
This creates mobile ions that are dispersed throughout the water, resulting in a homogeneous mixture.
However, there are important exceptions to note:

• Chlorides and bromides of metals like silver (Ag extsuperscript{+}), lead (Pb extsuperscript{2+}), and mercury (Hg extsubscript{2} extsuperscript{2+}) usually do not dissolve well in water.
• These metal ions form very strong ionic bonds with chlorides and bromides, which require a significant amount of energy to break.

For instance, compounds like sodium chloride (NaCl) and aluminum bromide (AlBr extsubscript{3}) are very soluble, except when paired with the aforementioned metals. This rule aids in predicting the behavior of chlorides and bromides in aqueous solutions.