15.2 Determining pH and Titrations

Summary

This video focuses on Chapter 15, Section 2, which covers the concepts of pH and titrations. It explains how pH indicators work, shifting between two forms based on the pH of a solution, causing a color change. The video also delves into titration, a method used to determine the concentration of an unknown solution by neutralizing it with a known acid or base. The equivalence point, where an acid and base neutralize each other, is a key concept, with the example demonstrating how to calculate the concentration of hydrochloric acid using sodium hydroxide.

Takeaways

• 😀 pH indicators work by breaking up into H+ ions and anions, shifting the equilibrium depending on the addition of acids or bases.
• 😀 When an acid is added to a pH indicator, it shifts the equilibrium to the left, as the excess H+ ions cause recombination with the anion.
• 😀 When a base is added to a pH indicator, the O- ions from the base combine with the indicator, shifting the equilibrium to the right.
• 😀 pH indicators only work within a specific pH range, known as the transition interval, where noticeable color changes occur.
• 😀 Titration is a technique used to determine the concentration of an unknown solution by neutralizing it with a solution of known concentration.
• 😀 Strong acids and bases, like HCl and NaOH, dissolve completely in solution, with their concentrations directly linked to the amounts of hydronium and hydroxide ions present.
• 😀 In a neutralization reaction, when an acid and base are in chemically equivalent amounts, they form neutral water with a pH of 7.
• 😀 The equivalence point in a titration for strong acids and bases always occurs at pH 7, marking the neutralization point.
• 😀 When titrating, the solution of known concentration is referred to as the standard solution or known solution.
• 😀 A sample calculation for titration involves using molarity and volume to calculate the concentration of an unknown acid or base solution, based on the neutralization reaction.

Q & A

What is the primary function of a pH indicator?

A pH indicator works by undergoing a reversible reaction that breaks down into hydrogen ions (H+) and an anion. The concentration of these ions shifts the equilibrium and changes the color of the indicator, signaling whether the solution is acidic or basic.

How does adding an acid or a base affect the equilibrium of a pH indicator?

When an acid is added to the solution, it increases the concentration of H+ ions, shifting the equilibrium to the left and recombining with the anion. Conversely, when a base is added, it increases the concentration of OH- ions, shifting the equilibrium to the right and breaking the indicator into its components.

Why do pH indicators only work within a specific pH range?

pH indicators only function within a certain pH range because they are designed to change color based on the concentration of hydrogen or hydroxide ions in solution. Outside of this range, the indicator cannot effectively show a color change.

What is the equivalence point in a titration?

The equivalence point in a titration is the point at which the amounts of acid and base in the solution are chemically equivalent, meaning the number of moles of H+ ions equals the number of moles of OH- ions. For strong acids and bases, this point occurs at a pH of 7.

How does titration allow the determination of the concentration of an unknown solution?

Titration involves adding a known concentration of an acid or base to a solution of unknown concentration. By monitoring the pH during the addition, and knowing the volume of the known solution required to neutralize the unknown solution, you can calculate the concentration of the unknown solution.

What happens to the pH when NaOH is added to hydrochloric acid during titration?

As NaOH is added to hydrochloric acid, the pH gradually increases. Once enough NaOH has been added to neutralize the acid, the pH spikes, reaching a point where the solution becomes basic.

In a titration involving hydrochloric acid (HCl) and sodium hydroxide (NaOH), what is the relationship between the moles of HCl and NaOH?

In a neutralization reaction between HCl and NaOH, the moles of HCl are equal to the moles of NaOH because the reaction between the acid and base occurs in a 1:1 molar ratio.

What is meant by 'standard solution' in the context of titration?

A standard solution is a solution of known concentration used in titration. For example, a known concentration of NaOH might be used to determine the concentration of an unknown HCl solution.

What is the molarity of a hydrochloric acid solution if 20 mL of 5 * 10^-3 M NaOH is required to neutralize 10 mL of it?

The molarity of the hydrochloric acid solution is 1 * 10^-2 M. This is calculated by using the molarity of NaOH and the volume used to neutralize the acid, assuming a 1:1 molar ratio in the neutralization reaction.

Why is it important to know the equivalence point in titration experiments?

Knowing the equivalence point is crucial in titration because it allows you to determine the exact moment when the acid and base are present in equal amounts, ensuring the accuracy of your calculation for the concentration of the unknown solution.