Problem 55 - Chemistry An Atoms First Approach

Question

What are the major species present in 0.250 \(M\) solutions of each of the following acids? Calculate the \(\mathrm{pH}\) of each of these solutions. a. \(\mathrm{HClO}_{4}\) b. HNO_3

Step-by-Step Solution

Answer

The major species present in the 0.250 M solutions of both perchloric acid (HClO₄) and nitric acid (HNO₃) are H⁺ and their corresponding anions, ClO₄⁻ and NO₃⁻, respectively. The pH of both solutions is approximately 0.60.

Step 1: Perchloric acid dissociates completely in water as follows: \[ \mathrm{HClO}_{4} \rightarrow \mathrm{H}^{+} + \mathrm{ClO}_{4}^{-} \] Since it is a strong acid, we assume 100% dissociation. #Step 2: Find the concentration of H₃O⁺ ions formed#

Our given initial concentration of HClO₄ is 0.250 M. Since the acid dissociates completely, the concentration of H⁺ ions formed in the solution will be equal to that of the HClO₄, which is 0.250 M. #Step 3: Calculate the pH of the solution#

Step 2: The pH of a solution is calculated using the following formula: \[ \mathrm{pH} = -\log[\mathrm{H}^{+}] \] Where [H⁺] is the concentration of H+ ions in the solution. Now, substitute the value of [H⁺] in the formula: \[ \mathrm{pH} = -\log(0.250) \] Use a calculator to find the pH: \[ \mathrm{pH} \approx 0.60 \] b. HNO₃ #Step 1: Write the dissociation equation of HNO₃#

Similarly, nitric acid dissociates completely in water as follows: \[ \mathrm{HNO}_{3} \rightarrow \mathrm{H}^{+} + \mathrm{NO}_{3}^{-} \] Assuming 100% dissociation due to the strong nature of the acid. #Step 2: Find the concentration of H₃O⁺ ions formed#

Step 3: Given initial concentration of HNO₃ is 0.250 M. Once again, the concentration of H⁺ ions formed in the solution is the same as that of the acid, which is 0.250 M. #Step 3: Calculate the pH of the solution#

Use the same pH formula as before: \[ \mathrm{pH} = -\log[\mathrm{H}^{+}] \] Substitute the value of [H⁺] in the formula: \[ \mathrm{pH} = -\log(0.250) \] Using a calculator, find the approximate pH: \[ \mathrm{pH} \approx 0.60 \] In conclusion, the major species present in the solutions are the corresponding ions of the acids, and the pH of both the 0.250 M HClO₄ and 0.250 M HNO₃ solutions is approximately 0.60.

Key Concepts

Understanding Strong Acids

Strong acids play a crucial role in chemistry due to their unique characteristics. A strong acid is defined as an acid that completely dissociates in water. This means when you dissolve it in water, it fully separates into its constituent ions. Consider it like breaking apart a LEGO structure completely, where nothing is left attached.
For example, when you dissolve perchloric acid ( HClO₄) in water, it dissociates completely to form hydrogen ions ( H⁺) and perchlorate ions ( ClO₄⁻). The term "strong acid" indicates that such dissociation occurs at 100% efficiency.

• Complete dissociation increases the concentration of hydrogen ions in a solution.
• This leads to a lower pH value, typically less than 3 for strong acids.

Understanding strong acids is key to calculating pH accurately and predicting reaction behavior in solutions.

The Process of Dissociation

Dissociation refers to the process where molecules split into smaller particles, ions in particular, when mixed with a solvent like water. In strong acids, this process is straightforward and complete.
For instance, analyzing the dissociation of perchloric acid (HClO₄), the reaction looks like this: \[ \mathrm{HClO}_{4} \rightarrow \mathrm{H}^{+} + \mathrm{ClO}_{4}^{-} \]

• This formula shows that for every molecule of HClO₄, we get one H⁺ ion and one ClO₄⁻ ion.
• The same principle applies to nitric acid (HNO₃) which dissociates to form H⁺ and NO₃⁻ ions.

Complete dissociation is what gives strong acids their potent characteristics, heavily influencing the pH of solutions.

Exploring Perchloric Acid and its Properties

Perchloric acid, with the chemical formula HClO₄, is one of the strongest acids known. It is highly soluble in water and dissociates completely into hydrogen ions ( H⁺) and perchlorate ions ( ClO₄⁻).
When working with perchloric acid, it is essential to remember:

• It is clear and colorless, like water.
• Because of its complete dissociation, solutions of HClO₄ have very low pH values.
• Handling HClO₄ requires caution due to its high reactivity and potential explosiveness when mixed with organic materials.

Knowing these properties is crucial for safely using perchloric acid in labs and understanding its chemical behavior.

Understanding Nitric Acid

Nitric acid ( HNO₃) is another strong acid that finds common use in laboratories due to its widespread applicability. Like perchloric acid, nitric acid fully dissociates in water into hydrogen ions ( H⁺) and nitrate ions ( NO₃⁻).
A few essential points to remember about nitric acid:

• HNO₃ appears as a colorless liquid that can turn yellow over time due to decomposition.
• It is used to manufacture fertilizers, and explosives, and in various industrial processes.
• Its strong oxidizing properties make it useful in etching and cleaning metals.

Understanding HNO₃’s dissociation and characteristics helps you comprehend its role in both chemical reactions and practical applications.