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Ch.17 - Acids and Bases
Chapter 17, Problem 137

Determine the pH of each solution. a. 0.0100 M HClO4 b. 0.115 M HClO2 c. 0.045 M Sr(OH)2 d. 0.0852 M KCN e. 0.155 M NH4Cl

Verified step by step guidance
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Step 1: Identify the nature of each compound in the solutions. HClO_4 is a strong acid, HClO_2 is a weak acid, Sr(OH)_2 is a strong base, KCN is a salt that can form a basic solution, and NH_4Cl is a salt that can form an acidic solution.
Step 2: For the strong acid HClO_4, calculate the pH directly from its concentration. Since it dissociates completely, [H^+] = 0.0100 M. Use the formula pH = -log[H^+].
Step 3: For the weak acid HClO_2, use the acid dissociation constant (K_a) to find [H^+]. Set up the equilibrium expression for the dissociation of HClO_2 and solve for [H^+]. Then, calculate the pH using pH = -log[H^+].
Step 4: For the strong base Sr(OH)_2, calculate the pOH first. Since Sr(OH)_2 dissociates completely and provides 2 OH^- ions per formula unit, [OH^-] = 2 * 0.045 M. Use pOH = -log[OH^-] and then find pH using pH = 14 - pOH.
Step 5: For the salts KCN and NH_4Cl, determine the pH by considering the hydrolysis of the ions. For KCN, CN^- will hydrolyze to form OH^-, making the solution basic. For NH_4Cl, NH_4^+ will hydrolyze to form H^+, making the solution acidic. Use the appropriate equilibrium expressions and constants (K_b for CN^- and K_a for NH_4^+) to find the pH.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Strong vs. Weak Acids and Bases

Strong acids, like HClO4, completely dissociate in water, leading to a straightforward calculation of pH based on molarity. In contrast, weak acids, such as HClO2, only partially dissociate, requiring the use of the acid dissociation constant (Ka) to determine the concentration of hydrogen ions and thus the pH.
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Strong vs Weak Bases

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 to 14. A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution. The pH is calculated using the formula pH = -log[H+], where [H+] is the concentration of hydrogen ions in the solution.
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Salt Hydrolysis

Salts formed from weak acids and strong bases, like KCN, can undergo hydrolysis in water, affecting the pH of the solution. For example, KCN dissociates into K+ and CN-, where CN- can react with water to produce OH- ions, resulting in a basic solution. Understanding this concept is crucial for calculating the pH of solutions containing salts.
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