Question

The salt \(\mathrm{BX}\) , when dissolved in water, produces an acidic solution. Which of the following could be true? (There may be more than one correct answer.) a. The acid HX is a weak acid. b. The acid HX is a strong acid. c. The cation \(B^{+}\) is a weak acid. Explain.

Short answer

The salt BX, when dissolved in water, produces an acidic solution. The possible scenarios that could be true are: a. The acid HX is a weak acid. c. The cation \(B^{+}\) is a weak acid.

Step-by-step solution

Option a: The acid HX is a weak acid

A weak acid, HX, does not fully ionize in water, producing only a small amount of [H+] ions. If the salt BX was derived from a weak acid, its dissolution in water will result in an acidic solution due to the partial ionization of the acid. Therefore, option a could be true.

Option b: The acid HX is a strong acid

A strong acid, HX, fully ionizes in water, producing a high concentration of [H+] ions. However, if the salt BX was derived from a strong acid, it will be neutralized by the conjugate base to form a neutral or slightly basic solution. In this case, the salt BX would not produce an acidic solution upon dissolution in water. Therefore, option b is not true.

Option c: The cation \(B^{+}\) is a weak acid

If the cation \(B^{+}\) from the salt BX is a weak acid, it will ionize in water to form another acidic species, such as Equation: \(B^{+} + H_2O \rightleftharpoons HB + OH^{-}\). This will increase the [H+] ions' concentration in the solution, making it acidic. Therefore, option c could be true. In conclusion, the exercise states that the salt BX, when dissolved in water, produces an acidic solution. The possible scenarios that could be true given the information are: a. The acid HX is a weak acid. c. The cation \(B^{+}\) is a weak acid.

Key concepts

Weak Acids

Weak acids are a class of acids that do not completely ionize in water. Unlike strong acids, weak acids only release a small portion of their hydrogen ions ([H^+]) into the solution. This partial ionization occurs because the acid molecules prefer to stay intact rather than dissociate entirely.
Weak acids create an equilibrium between the ionized and unionized form, written as:\[HA(aq) \rightleftharpoons H^+(aq) + A^-(aq)\]where \(HA\) represents the weak acid, \(H^+\) the hydrogen ion, and \(A^-\) the conjugate base.
Common examples include acetic acid (CH₃COOH) and hydrofluoric acid (HF).- They have a higher pH compared to strong acids because of the fewer hydrogen ions in the solution.- The equilibrium constant, Ka, is used to represent the strength of a weak acid.
In this situation, when you dissolve a salt made from a weak acid like HX, you often end up with an acidic solution because there are still some unfavored A- ions that react with water to recreate the weak acid, dissociating slightly and increasing [H^+] in the solution.

Salt Dissolution

Salt dissolution refers to the process where a salt, composed of positive and negative ions, dissolves in water. When a salt dissolves, it separates into its respective ions. However, not all solutions formed by dissolving a salt are neutral. Some can be acidic or basic, depending on the nature of the ions.
A salt can result from the reaction of a weak acid and a strong base, or vice-versa. For example, the dissolution of the salt BX in water reveals the nature of its parent acid and base.
- If the parent acid HX is weak, its conjugate base X^- is relatively strong. This enables X^- to hydrolyze water, react, and somewhat offset the neutrality—making the solution acidic:\[X^- + H_2O \rightleftharpoons HX + OH^-\]- Likewise, if the cation B^+ is a weak acid, it hydrolyzes, contributing additional [H^+], making the solution acidic. Understanding salt dissolution helps identify why certain salts lead to acidic or basic solutions when mixed with water. The composition of the salt and its hydrolysis reaction are crucial factors.

Chemical Ionization

Chemical ionization involves the dissociation of a compound into ions when dissolved in a solvent like water. For any compound, especially those contributing acidity in solution, how it ionizes determines the behavior and properties of the solution.
When it comes to weak acids, ionization is incomplete, creating an equilibrium mixture of ions and undissolved acid molecules. Conversely, strong acids completely ionize, leaving no non-ionized acid in the solution.
- In the dissolution of the salt BX , the extent of ionization of its components can shift the pH.
- If HX is a weak acid, ionization involves the conversion of its conjugate base X^- back to HX , introducing H^+ ions.
- For a situation where B^+ acts as a weak acid, it partially ionizes, contributing to additional H^+ ions.
Ionization processes are dictated by the strength of acids or bases involved and are characterized by equilibrium constants. Chemical ionization is central to predicting and understanding the changes in pH of solutions containing dissolved salts.