Problem 19 - Chemistry The Central Science

Question

Using solubility guidelines, predict whether each of the following compounds is soluble or insoluble in water: (a) \(\mathrm{MgBr}_{2}\), (b) \(\mathrm{PbI}_{2}\), (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}\) (e) \(\mathrm{ZnSO}_{4}\). (d) \(\mathrm{Sr}(\mathrm{OH})_{2}\),

Step-by-Step Solution

Answer

Using the solubility guidelines, we can predict the solubility of the given compounds as follows: (a) \(\mathrm{MgBr}_{2}\) is soluble in water. (b) \(\mathrm{PbI}_{2}\) is insoluble in water. (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}\) is soluble in water. (e) \(\mathrm{ZnSO}_{4}\) is soluble in water. (d) \(\mathrm{Sr}(\mathrm{OH})_{2}\) is insoluble in water.

Step 1: Review solubility guidelines

Remember these general solubility guidelines: 1. Most salts containing Group 1 elements (Li, Na, K, Rb, Cs) and the ammonium ion (NH4+) are soluble. 2. Most nitrates (NO3-), acetates (CH3COO-), and perchlorates (ClO4-) are soluble. 3. Most chlorides (Cl-), bromides (Br-), and iodides (I-) are soluble, except for those of silver (Ag+), lead (Pb2+), and mercury(I) (Hg2_2^2+). 4. Most sulfates (SO4^2-) are soluble, except for those of barium (Ba^2+), lead (Pb^2+), calcium (Ca^2+), and silver (Ag^+). 5. Most carbonates (CO3^2-), phosphates (PO4^3-), sulfides (S^2-), and hydroxides (OH-) are insoluble, except for those of Group 1 elements and the ammonium ion. Now, let's use these guidelines to determine the solubility of each compound in water.

Step 2: Determine the solubility of MgBr2

MgBr2 consists of the magnesium ion (Mg^2+) and the bromide ion (Br-). According to rule 3 of our solubility guidelines, most bromides are soluble in water. There are no exceptions mentioned for magnesium, which means that magnesium bromide (MgBr2) is soluble in water.

Step 3: Determine the solubility of PbI2

PbI2 consists of lead ion (Pb^2+) and iodide ion (I-). According to rule 3, most iodides are soluble, except for those of silver, lead, and mercury(I). Since PbI2 contains the lead ion, it is an exception to the rule and is therefore insoluble in water.

Step 4: Determine the solubility of (NH4)2CO3

(NH4)2CO3 consists of the ammonium ion (NH4+) and the carbonate ion (CO3^2-). According to rule 1, most compounds containing the ammonium ion are soluble. None of the other rules mention an exception for ammonium carbonate. Therefore, ammonium carbonate ((NH4)2CO3) is soluble in water.

Step 5: Determine the solubility of ZnSO4

ZnSO4 consists of the zinc ion (Zn^2+) and the sulfate ion (SO4^2-). According to rule 4, most sulfates are soluble in water, except for those of barium, lead, calcium, and silver. Since zinc is not mentioned as an exception, zinc sulfate (ZnSO4) is soluble in water.

Step 6: Determine the solubility of Sr(OH)2

Sr(OH)2 consists of the strontium ion (Sr^2+) and the hydroxide ion (OH-). According to rule 5, most hydroxides are insoluble in water, except for those of Group 1 elements and the ammonium ion. Strontium is not a Group 1 element nor the ammonium ion, so strontium hydroxide (Sr(OH)2) is insoluble in water. In conclusion: (a) \(\mathrm{MgBr}_{2}\) is soluble in water. (b) \(\mathrm{PbI}_{2}\) is insoluble in water. (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}\) is soluble in water. (e) \(\mathrm{ZnSO}_{4}\) is soluble in water. (d) \(\mathrm{Sr}(\mathrm{OH})_{2}\) is insoluble in water.

Key Concepts

Soluble and Insoluble Compounds

Understanding the difference between soluble and insoluble compounds is a fundamental step in the study of chemistry. A soluble compound dissolves in a solvent, forming a homogeneous mixture or solution. In contrast, an insoluble compound does not dissolve in a solvent, instead remaining separate.

For example, table salt ( NaCl) is soluble in water, meaning it can dissolve to form a clear solution. On the other hand, sand ( SiO2) is insoluble in water and will settle at the bottom of a container. The solubility of a compound is influenced by factors such as temperature, the nature of the solvent and solute, and the ionic strength of the solution.

When considering aqueous solutions, the polarity of water molecules is important. Polar water molecules can interact with ionic compounds and polar covalent molecules, aiding in solubilization. Furthermore, pressure and the presence of other substances can also affect solubility.

Solubility Rules in Chemistry

Learning the solubility rules in chemistry helps you predict the solubility of ionic compounds in water. These rules are based on empirical observations and can often be summarized in a list of guidelines that help students make quick predictions regarding solubility. The solubility rules as mentioned in the exercise solution provide a framework for identifying which compounds will dissolve in water and which will not.

Understanding the solubility rules requires recognition of ion charges and the ability to identify patterns among different groups of elements. Rules for common ions, such as nitrates and acetates being soluble, or the exceptions for sulfates with calcium or barium, are particularly valuable for solving problems in chemistry. It is important to remember these rules as they apply to most situations, though there might be exceptions under specific conditions.

The rules not only facilitate homework exercises but also have real-world applications in fields such as pharmaceuticals, where predicting the solubility of drugs is crucial.

Predicting Compound Solubility

The task of predicting compound solubility is a practical application of solubility rules and guidelines. By using a structured approach, one can often determine whether a compound is likely to be soluble or not. The exercise provided illustrates this process through a series of steps.

Familiarity with the periodic table and the common ions is necessary for applying these guidelines effectively. For instance, knowing that Group 1 elements form soluble compounds allows for a quick determination of the solubility of compounds like sodium chloride ( NaCl). Recognizing that lead ( Pb^2+) forms an exception to the solubility of iodides enables the prediction that lead iodide ( PbI2) is insoluble.

Consistently using these guidelines helps with memorization, but more importantly, it cultivates an understanding of ionic interactions in aqueous solutions. Applying these rules systematically, as shown in the solution, reinforces competency in predicting solubility, which is a skill that transcends classroom learning and can be applied in various scientific situations.