What mass of HClO4 must be present in 0.500 L of solution to obtain a solution with each pH value? a. pH = 2.50 b. pH = 1.50 c. pH = 0.50
Ch.17 - Acids and Bases
Chapter 17, Problem 64
Determine the [H3O+] and pH of a 0.200 M solution of formic acid.

1
Identify the chemical formula for formic acid, which is HCOOH.
Write the dissociation equation for formic acid in water: \( \text{HCOOH} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{HCOO}^- \).
Use the acid dissociation constant \( K_a \) for formic acid, which is typically given or can be found in a reference table.
Set up an ICE (Initial, Change, Equilibrium) table to determine the concentrations of the species at equilibrium.
Use the expression for \( K_a = \frac{[\text{H}_3\text{O}^+][\text{HCOO}^-]}{[\text{HCOOH}]} \) to solve for \([\text{H}_3\text{O}^+]\), and then calculate the pH using \( \text{pH} = -\log[\text{H}_3\text{O}^+] \).

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid-Base Chemistry
Acid-base chemistry involves the study of substances that can donate protons (H<sup>+</sup>) or accept protons. In this context, formic acid (HCOOH) is a weak acid that partially dissociates in water to produce H<sub>3</sub>O<sup>+</sup> ions. Understanding the behavior of weak acids is crucial for calculating the concentration of hydronium ions in solution.
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Dissociation Constant (K<sub>a</sub>)
The dissociation constant (K<sub>a</sub>) quantifies the strength of an acid in solution. For formic acid, K<sub>a</sub> is used to determine the extent of its dissociation into H<sub>3</sub>O<sup>+</sup> and its conjugate base (HCOO<sup>-</sup>). This constant is essential for calculating the equilibrium concentrations of the species in the solution.
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pH Calculation
pH is a measure of the acidity of a solution, defined as the negative logarithm of the hydronium ion concentration: pH = -log[H<sub>3</sub>O<sup>+</sup>]. To find the pH of the formic acid solution, one must first determine the concentration of H<sub>3</sub>O<sup>+</sup> using the dissociation constant and then apply the pH formula. This relationship is fundamental in acid-base chemistry.
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