Worked example: Calculating the pH after a weak acid–strong base reaction (excess acid)

TL;DR

The video explains how to calculate the pH of a solution resulting from the reaction between a weak acid (acetic acid) and a strong base (sodium hydroxide).

Transcript

• [Instructor] Let's look at a reaction between a weak acid, acetic acid, and a strong base, sodium hydroxide. Let's say we have 100 milliliters of a 2.0 molar solution of aqueous acetic acid, and that's mixed with 100 milliliters of a 1.0 molar solution of aqueous sodium hydroxide. Our goal is to find the pH of the resulting solution at 25 degrees... Read More

Key Insights

• 🥺 The reaction between a weak acid and a strong base undergoes complete neutralization, leading to the formation of products.
• 🚰 An ICF table is used instead of an ICE table to track the changes in moles of reactants and products.
• ⚾ Concentrations of acetic acid and acetate anions are calculated based on the moles and the total volume of the solution.
• 😑 The pH of the resulting solution is determined using the concentration of hydronium ions, obtained from the equilibrium concentration of the weak acid.

Questions & Answers

Q: What is the initial step in finding the pH of a solution formed from a weak acid-strong base reaction?

The initial step involves calculating the moles of the weak acid and strong base present by using the molarity equation (Molarity = Moles/Liters).

Q: Why is there an arrow going to the right instead of an equilibrium arrow in the net ionic equation?

The arrow going to the right signifies that the neutralization reaction between the weak acid and strong base goes to completion, resulting in the formation of products.

Q: How are the concentrations of acetic acid and acetate anions determined?

By using the moles of acetic acid and acetate anions along with the total volume of the solution, the concentrations are calculated using the molarity equation (Molarity = Moles/Volume).

Q: How is the pH of the resulting solution calculated?

The pH is calculated by taking the negative logarithm (base 10) of the concentration of hydronium ions, which is obtained from the equilibrium concentration of the weak acid.

Summary & Key Takeaways

• The video discusses the steps to determine the pH of a solution formed from the reaction between acetic acid and sodium hydroxide.

• Initial calculations are done to find the number of moles of the weak acid and strong base present.

• A net ionic equation is used to represent the reaction, and an ICF table is set up to calculate the changes in moles of reactants and products.

• Concentrations of acetic acid and the acetate anion are determined, leading to the calculation of the solution's pH.