16.5 pH Calculations for Weak Acids and Bases | General Chemistry

The video begins with an introduction to the topic of pH calculations for weak acids and bases. The instructor, Chad, welcomes viewers to 'Chad's Prep' and outlines the services offered, including high school and college science prep, as well as MCAT, DAT, and OAT preparation. He introduces the lesson as part of a general chemistry playlist and encourages viewers to subscribe for updates. The lesson focuses on the differences between strong and weak acids, highlighting that weak acids only partially dissociate, leading to a pH greater than one. Chad explains the need for additional information, such as the Ka expression and value, to determine the exact pH of a weak acid solution.

This paragraph delves deeper into the concept of the equilibrium constant (Ka) for weak acids. Chad explains that the strength of an acid is measured by its degree of dissociation, with a higher dissociation leading to a higher Ka value. He provides examples of three weak acids with their respective Ka values, allowing viewers to compare their relative strengths. The discussion transitions into how to calculate the pH of a weak acid solution using the Ka value and the initial concentration of the acid. Chad emphasizes the complexity of this process compared to strong acids but promises to introduce a shortcut method for most weak acids.

Chad presents a step-by-step process for calculating the pH of a 0.1 molar solution of acetic acid as an example. He introduces the generic Ka expression and explains how to set up an ICE (Initial, Change, Equilibrium) table to determine the equilibrium concentration of H+ ions. The calculation involves simplifying the Ka expression by ignoring the minus X term due to the small amount of dissociation in weak acids. Chad demonstrates how to solve for X, which represents the equilibrium concentration of H+, and then how to use this value to calculate the pH. He also mentions that a shortcut exists for such calculations, which will be discussed later in the lesson.

In this section, Chad addresses a different scenario where the pH of a weak acid solution is known, and the goal is to find the Ka value. Using the pH value, he explains how to determine the equilibrium concentration of H+ ions and subsequently calculate the Ka value. He reiterates the process of setting up the ICE table and solving the Ka expression, but this time including the minus X term for accuracy. Chad then calculates the Ka value and discusses the percentage dissociation of the weak acid, providing a method to determine how much of the acid has dissociated based on the initial concentration and the equilibrium concentration of H+.

Chad transitions from weak acids to weak bases, explaining the difference in their dissociation reactions and the involvement of water. He introduces the equilibrium constant for weak bases, known as Kb, and explains its relationship with the Ka of the conjugate acid. The discussion includes the Bronsted-Lowry definition of a weak base as a proton acceptor and the necessity of including water in the reaction equation. Chad sets up the generic reaction for a weak base dissociating in water and introduces the Kb expression, drawing parallels to the process followed for weak acids.

Chad demonstrates how to calculate the pH of a weak base solution, specifically a 0.1 molar solution of ammonia with a given Kb value. He outlines the process of setting up an ICE table for the weak base and solving for the hydroxide ion (OH-) concentration. The calculation involves a similar approach to that used for weak acids but with a focus on OH- instead of H+. Chad shows how to solve for X, representing the hydroxide concentration, and then how to convert this to POH and subsequently calculate the pH by subtracting from 14. He emphasizes the extra step required for weak bases compared to weak acids and reiterates the process for clarity.

Chad tackles a more challenging problem: finding the Kb value of a weak base when the pH of a 0.1 molar solution is known. He guides viewers through the process of converting the pH to POH, determining the hydroxide ion concentration, and then using this information to solve for Kb. The calculation requires substituting the known values into the Kb expression and accounting for the minus X term. Chad calculates the Kb value, highlighting the weak nature of the base in question. He reinforces the importance of understanding the differences in calculating pH for weak acids and bases and the relationship between their respective Ka and Kb values.

In the concluding paragraph, Chad summarizes the key points from the lesson, emphasizing the importance of understanding the different types of solutions and their corresponding pH calculation methods. He outlines the processes for calculating the pH of strong acids and bases, weak acids and bases, and hints at upcoming topics such as buffers and titrations. Chad encourages viewers to practice and master pH calculations, providing resources for further study and practice. He also invites viewers to engage with the content by using the thumbs up button and explores additional study materials through the provided links.