Skip to main content
Ch.17 - Acids and Bases
Chapter 17, Problem 64

Determine the [H3O+] and pH of a 0.200 M solution of formic acid.

Verified step by step guidance
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 \( \text{pH} = -\log[\text{H}_3\text{O}^+] \).

Verified Solution

Video duration:
3m
This video solution was recommended by our tutors as helpful for the problem above.
Was this helpful?

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.
Recommended video:
Guided course
02:00
Arrhenius Acids and Bases

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.
Recommended video:
Guided course
01:14
Equilibrium Constant K

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.
Recommended video:
Guided course
01:30
pH Calculation Example