Problem 100 - General Chemistry: Principles and Modern Applications

Question

Which of the following solids are likely to be more soluble in acidic solution and which in basic solution? Which are likely to have a solubility that is independent of pH? Explain. (a) \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4} ;\) (b) \(\mathrm{MgCO}_{3} ;\) (c) \(\mathrm{CdS}\); (d) \(\mathrm{KCl} ;\) (e) \(\mathrm{NaNO}_{3} ;\) (f) \(\mathrm{Ca}(\mathrm{OH})_{2}\).

Step-by-Step Solution

Answer

Compound (a) H2C2O4 and (f) Ca(OH)2 are more soluble in basic and acidic environment respectively. Compounds (b) MgCO3 and (c) CdS are more soluble in acidic solution. Compounds (d) KCl and (e) NaNO3 have solubility independent of pH.

Step 1: Analyzing Compound (a) - H2C2O4

H2C2O4 is oxalic acid, a diatomic acid. It will be more soluble in a basic environment, as the base can accept the protons that the acid donates, creating water and a salt, thereby driving the dissolution process forward.

Step 2: Analyzing Compound (b) - MgCO3

MgCO3 is a basic substance due to the CO3 (carbonate) ion. It will be more soluble in an acidic environment since it reacts with acid to neutralize it, forming water and a salt.

Step 3: Analyzing Compound (c) - CdS

CdS is a somewhat special case. It is usually not very soluble, but its solubility can be increased in an acidic medium due to the formation of soluble species like Cd2+ and HS-, driven by the reaction with H+ ions from the acid.

Step 4: Analyzing Compound (d) - KCl

KCl is a salt. It is expected to be equally soluble in both acidic and basic solutions as it readily dissociates into K+ and Cl- ions in any aqueous solution. Therefore, its solubility is independent of pH.

Step 5: Analyzing Compound (e) - NaNO3

NaNO3 is also a salt and will readily dissociate in any aqueous solution, making it equally soluble in both acidic and basic solutions. Therefore, its solubility is independent of pH too.

Step 6: Analyzing Compound (f) - Ca(OH)2

Ca(OH)2 is a strong base. It will be more soluble in an acidic solution as the acid will react with the base to form water and a salt, thereby driving the solubility.

Key Concepts

Acid-Base Reactions

Understanding acid-base reactions is crucial when examining the solubility of different compounds in various pH environments. At its simplest, an acid can be thought of as a substance that donates protons (H+ ions), and a base is one that accepts them. When an acid and base come into contact, they react in a process known as neutralization to form water and a salt.

For example, a basic substance like MgCO3 will dissolve in an acidic solution. The reaction will form water and a salt, which usually increases the compound's solubility as the resulting ions are more soluble than the intact compound. Similarly, an acidic substance, such as oxalic acid (H2C2O4), becomes more soluble in a basic solution, as the base neutralizes the acid.

pH-Dependence of Solubility

The solubility of certain compounds is highly dependent on the pH of their environment. pH measures the concentration of H+ ions in a solution, and this directly impacts the solubility of compounds that can react with those ions.

For example, Ca(OH)2 is more soluble in an acidic environment because the acid provides a source of H+ ions to react with the hydroxide ions, forming water. This process reduces the concentration of OH- ions, which shifts the equilibrium towards more dissolution of Ca(OH)2. On the other hand, substances like KCl and NaNO3, which don't partake in acid-base reactions, will dissolve regardless of the pH, showcasing solubility that is pH-independent.

Compound Solubility Analysis

When analyzing the solubility of compounds, it's important to identify the nature of each substance—whether it's an acid, base, or salt. Acidic and basic compounds generally increase in solubility when paired with a substance that can neutralize them. For example, CdS has low solubility but can react with acids to form soluble products.

To predict solubility behavior in different pH environments, it's helpful to consider the potential reactions that a substance might undergo. A compound such as CdS reacts with the H+ ions in acid, increasing its solubility. However, salts that do not react with H+ or OH- ions, like KCl and NaNO3, will not experience changes in solubility with changes in pH. This type of compound solubility analysis is essential for making informed predictions about solubility under various conditions.