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Chapter 16: Problem 69

Predict the \(\mathrm{pH}(>7,<7,\) or \(\approx 7)\) of the aqueous solutions containing the following salts: (a) \(\mathrm{KBr}\) (b) \(\mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}\) (c) \(\mathrm{BaCl}_{2}\) (d) \(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\)

Short Answer

Expert verified
(a) Neutral, (b) Acidic, (c) Neutral, (d) Acidic.

Step by step solution

01

Determine the nature of each ion

To predict the pH of a salt solution, we start by identifying the possible reactions of the ions with water. For each salt, dissociate it into its respective cations and anions. - **(a)** KBr dissociates into K鈦 and Br鈦. K鈦 is a neutral cation from a strong base (KOH), and Br鈦 is a neutral anion from a strong acid (HBr). - **(b)** Al(NO鈧)鈧 dissociates into Al鲁鈦 and NO鈧冣伝. NO鈧冣伝 is a neutral anion from a strong acid (HNO鈧), and Al鲁鈦 is a cation that can hydrolyze. - **(c)** BaCl鈧 dissociates into Ba虏鈦 and Cl鈦. Ba虏鈦 is a neutral cation from a strong base (Ba(OH)鈧), and Cl鈦 is a neutral anion from a strong acid (HCl). - **(d)** Bi(NO鈧)鈧 dissociates into Bi鲁鈦 and NO鈧冣伝. Bi鲁鈦 can hydrolyze, while NO鈧冣伝 is neutral.
02

Predict the effect of cations and anions on solution pH

After dissociation, analyze the ions to assess the overall effect on the pH: - **(a) KBr:** K鈦 and Br鈦 are both from strong acid and base, meaning they don't affect pH. The solution will have a **neutral pH (鈮7)**. - **(b) Al(NO鈧)鈧:** Al鲁鈦 is a small, highly charged cation which can hydrolyze in water to produce H鈦 ions, making the solution **acidic (pH<7)**. - **(c) BaCl鈧:** Ba虏鈦 and Cl鈦 are from strong acid and base, so they don't affect the pH. The solution will remain **neutral (鈮7)**. - **(d) Bi(NO鈧)鈧:** Bi鲁鈦, similar to Al鲁鈦, will hydrolyze in water generating H鈦 ions, making the solution **acidic (pH<7)**.
03

Confirm pH predictions based on ion interactions

Al鲁鈦 and Bi鲁鈦 can increase the acidity of the solution through hydrolysis: - Al鲁鈦: Al鲁鈦 + 3H鈧侽 鈬 Al(OH)鈧 + 3H鈦, increasing [H鈦篯 and lowering the pH. - Bi鲁鈦: Bi鲁鈦 + 3H鈧侽 鈬 Bi(OH)鈧 + 3H鈦, increasing [H鈦篯 and lowering the pH. Thus, (b) Al(NO鈧)鈧 and (d) Bi(NO鈧)鈧 solutions are acidic, while (a) KBr and (c) BaCl鈧 remain neutral.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cation Hydrolysis
When certain cations dissolve in water, they can undergo a process called cation hydrolysis, wherein the cation reacts with water molecules. This interaction typically generates hydrogen ions, contributing to the acidity of the solution.
  • For example, cations like \( ext{Al}^{3+}\) and \( ext{Bi}^{3+}\) have high positive charges and small ionic radii, making them highly polarizing. This results in hydrolysis reactions such as: \[\text{Al}^{3+} + 3\text{H}_2\text{O} \leftrightarrow \text{Al(OH)}_3 + 3\text{H}^+\] This process contributes to an increase in \(\text{H}^+\) concentration, leading to a lower pH.
  • Cations from strong bases, such as \( ext{K}^+\) from \(\text{KOH}\), do not hydrolyze. They do not alter the pH, preserving the neutrality of solutions like \(\text{KBr}\).
Understanding which cations can hydrolyze helps predict whether a solution will be acidic or neutral.
pH Prediction
Determining the pH of salt solutions requires evaluating the potential effect of each ion, derived from the salt's dissociation, on the hydrogen ion concentration.
For instance, if both the cation and anion originate from strong acids and bases (like in \( ext{KBr}\) and \( ext{BaCl}_2\)), they do not change the pH, maintaining a neutral environment (\(\text{pH} \approx 7\)).
  • Cations such as \(\text{Al}^{3+}\) and \(\text{Bi}^{3+}\), however, are noteworthy because they can undergo hydrolysis, incrementing the \(\text{H}^+\) presence, thus predicting an acidic (\(\text{pH} < 7\)) outcome.
Hence, predicting pH involves recognizing these patterns of ionic interaction.
Neutralization of Strong Acids and Bases
The neutral character of solutions formed by strong acids and bases is a result of the complete ionic dissociation these compounds undergo in water.
A salt like \(\text{KBr}\) illustrates this point well. Derived from a strong acid (\(\text{HBr}\)) and a strong base (\(\text{KOH}\)), when dissociated in water, \(\text{K}^+\) and \(\text{Br}^-\) do not engage in reactions affecting the hydrogen ion concentration.
This balance typically results in a neutral pH.
  • For \(\text{BaCl}_2\), the rationale is similar: both \(\text{Ba}^{2+}\) and \(\text{Cl}^-\) ions do not impose any further pH changes, maintaining neutrality. This is commonly true when salts are made from strong acids and strong bases.
Recognizing these scenarios is essential for understanding why some solutions remain neutral even after dissolution.
Ionic Dissociation
In water, salts dissociate into their respective cations and anions. This process is known as ionic dissociation and plays a crucial role in determining the resultant pH of the solution.
During dissociation, the individual ions separate and diffuse throughout the solution, allowing them to potentially interact with water molecules.
  • For example, \(\text{KBr}\) dissociates to form \(\text{K}^+\) and \(\text{Br}^-\).
  • Similarly, \(\text{Al(NO}_3)_3\) dissociates into \(\text{Al}^{3+}\) and \(\text{NO}_3^-\) ions.
While some ions, like \(\text{NO}_3^-\) or \(\text{Br}^-\), typically don鈥檛 contribute to pH changes, their cations' ability to hydrolyze can dictate whether the solution will become acidic. Thus, knowing the dissociation products is vital for gauging a salt solution's pH.

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Most popular questions from this chapter

(a) Calculate the percent ionization of a \(0.20 \mathrm{M}\) solution of the monoprotic acetylsalicylic acid (aspirin). \(\left(K_{\mathrm{a}}=3.0 \times 10^{-4} .\right)\). (b) The \(\mathrm{pH}\) of gastric juice in the stomach of a certain individual is \(1.00 .\) After a few aspirin tablets have been swallowed, the concentration of acetylsalicylic acid in the stomach is \(0.20 \mathrm{M}\). Calculate the percent ionization of the acid under these conditions.

Teeth enamel consists largely of hydroxyapatite \(\left[\mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH}\right] .\) When it dissolves in water (a process called demineralization), it dissociates as follows: $$ \mathrm{Ca}_{5}\left(\mathrm{PO}_{4}\right)_{3} \mathrm{OH} \longrightarrow 5 \mathrm{Ca}^{2+}+3 \mathrm{PO}_{4}^{3-}+\mathrm{OH}^{-} $$ The reverse process, called remineralization, is the body's natural defense against tooth decay. Acids produced from food remove the \(\mathrm{OH}^{-}\) ions and thereby weaken the enamel layer. Most toothpastes contain a flouride compound such as \(\mathrm{NaF}\) or \(\mathrm{SnF}_{2}\). What is the function of these compounds in preventing tooth decav?

Classify each of these species as a weak or strong base: (a) LiOH, (b) \(\mathrm{CN}^{-}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{ClO}_{4}^{-},\) (e) \(\mathrm{NH}_{2}\).

Consider the two weak acids HX (molar mass = \(180 \mathrm{~g} / \mathrm{mol}\) ) and HY (molar mass \(=78.0 \mathrm{~g} / \mathrm{mol}\) ). If a solution of \(16.9 \mathrm{~g} / \mathrm{L}\) of \(\mathrm{HX}\) has the same \(\mathrm{pH}\) as one containing \(9.05 \mathrm{~g} / \mathrm{L}\) of \(\mathrm{HY},\) which is the stronger acid?

Define \(\mathrm{pH}\). Why do chemists normally choose to discuss the acidity of a solution in terms of \(\mathrm{pH}\) rather than hydrogen ion concentration, \(\left[\mathrm{H}^{+}\right]?\)
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