Question

Predict whether aqueous solutions of the following subsstances are acidic, basic, or neutral: \((\mathbf{a})\mathrm{AlCl}_{3},\) \((\mathbf{b})\mathrm{NaBr}\) \((\mathbf{c}) \mathrm{NaClO},(\mathbf{d})\left[\mathrm{CH}_{3} \mathrm{NH}_{3}\right] \mathrm{NO}_{3},(\mathbf{e}) \mathrm{Na}_{2} \mathrm{SO}_{3}\)

Short answer

The final prediction for the pH of the substances is: a) AlCl3: Acidic b) NaBr: Neutral c) NaClO: Basic d) CH3NH3NO3: Acidic e) Na2SO3: Basic

Step-by-step solution

Dissociate the substances in water

When the compounds dissolve in water, they dissociate into their respective ions. Let's start by writing the dissociation equations for each substance. a) AlCl3 -> Al^3+ + 3 Cl^- b) NaBr -> Na^+ + Br^- c) NaClO -> Na^+ + ClO^- d) CH3NH3NO3 -> CH3NH3^+ + NO3^- e) Na2SO3 -> 2 Na^+ + SO3^2-

Analyze the properties of the ions

Next, we'll consider the properties of the dissociated ions to predict the pH of the solution. a) Al^3+ (cation) is acidic since it's an amphoteric ion and can react with water to form H^+ ions. Cl^- (anion) is the conjugate base of HCl, a strong acid, and it won't affect the pH. b) Na^+ (cation) won't affect the pH as it's the conjugate acid of NaOH, a strong base. Br^- (anion) is the conjugate base of HBr, a strong acid, and it won't affect the pH. c) Na^+ (cation) won't affect the pH as it's the conjugate acid of NaOH, a strong base. ClO^- (anion) is the conjugate base of a weak acid; therefore, it will produce OH^- ions, making the solution basic. d) CH3NH3^+ (cation) is the conjugate acid of a weak base, which will produce H^+ ions when it reacts with water, making the solution acidic. NO3^- (anion) is the conjugate base of HNO3, a strong acid, and won't affect the pH. e) Na^+ (cation) won't affect the pH as it's the conjugate acid of NaOH, a strong base. SO3^2- (anion) is the conjugate base of a weak acid, which will produce OH^- ions when it reacts with water, making the solution basic.

Determine the pH of the substances

Based on the properties of the ions formed in each solution, we can predict if the substances are acidic, basic, or neutral: a) AlCl3 -> Acidic b) NaBr -> Neutral c) NaClO -> Basic d) CH3NH3NO3 -> Acidic e) Na2SO3 -> Basic The final prediction for the pH of the substances is: a) AlCl3: Acidic b) NaBr: Neutral c) NaClO: Basic d) CH3NH3NO3: Acidic e) Na2SO3: Basic

Key concepts

pH prediction

Understanding the pH of a solution involves knowing whether it is acidic, basic, or neutral. The pH scale, ranging from 0 to 14, measures how acidic or basic a solution is. Solutions with a pH less than 7 are acidic, those with a pH of 7 are neutral, and solutions with a pH greater than 7 are considered basic.

When predicting the pH of solutions, we first look at the dissociation of the substance in water to form ions. By examining the nature of the resulting ions, we can infer the solution's pH. For instance, ions that are conjugate bases of strong acids, such as Cl^- from HCl, do not significantly affect the pH and often result in a neutral solution. Conversely, ions that are conjugate bases of weak acids, or conjugate acids of weak bases, can respectively increase the concentration of OH^- or H⁺ ions in the solution, leading to basic or acidic solutions, respectively. This approach integrates our knowledge of acid-base chemistry and the behavior of ions in water, allowing us to make educated predictions about pH.

Dissociation equations

Dissociation equations are vital for understanding how compounds behave when dissolved in water. They show the splitting of compounds into constituent ions, which is essential for predicting a solution's properties, including pH. As seen in substances like AlCl₃ and Na₂SO₃, the dissociation equations help identify the ions present in solution.

By establishing these dissociation equations, we can better evaluate how each ion contributes to the overall acidity or basicity of the solution. Starting with the dissociation equations is the first critical step in understanding acid-base behavior of solutions, setting the stage for deeper analysis such as identifying conjugate acid-base pairs and predicting the pH.

Conjugate acids and bases

Conjugate acids and bases are a pair of substances that differ by the presence of one H⁺ ion. They always appear in pairs: when an acid donates an H⁺, it becomes a conjugate base; when a base accepts an H⁺, it becomes a conjugate acid. For instance, H₂O and OH^- form a conjugate acid-base pair.

The concept of conjugate acid-base pairs is crucial for predicting the behavior of ions in water. Ions that are conjugate bases of strong acids (like Cl^- or NO₃^-) have little tendency to act as bases in water. On the other hand, ions that are conjugate bases of weak acids (such as SO₃^2- and ClO^-) will increase the solution's pH, making it basic. The knowledge of conjugate acids and bases helps students to understand why certain ions affect the acidic or basic nature of a solution.