Question

The equivalence point was reached in titrations of three unknown bases at \(\mathrm{pH} 5.53\) (base A), \(4.11\) (base B), and \(6.00\) (base C). (a) Which is the strongest base? (b) Which is the weakest base?

Short answer

(a) Strongest base: Base C (b) Weakest base: Base B

Step-by-step solution

Understanding the Concept

In a titration, the equivalence point is where the amount of titrant is stoichiometrically equivalent to the amount of analyte. For a base titration, the equivalence point \( ext{pH}\) provides information about the strength of the base. A higher \( ext{pH}\) at equivalence indicates a stronger base, as stronger bases have stronger conjugate acids, leading to a higher pH.

List the pH Values at Equivalence

For each base, write down the given pH value at the equivalence point: - Base A: \(5.53\)- Base B: \(4.11\)- Base C: \(6.00\)

Comparing pH Values

Compare the pH values at the equivalence points. The strongest base will have the highest pH at equivalence, and the weakest base will have the lowest pH.- Highest pH: \(6.00\) for Base C- Lowest pH: \(4.11\) for Base B

Identifying the Strongest and Weakest Base

Based on the pH comparisons: - Strongest base: Base C (pH 6.00) - Weakest base: Base B (pH 4.11)

Key concepts

Equivalence Point

The equivalence point in a titration marks the stage where the amount of titrant added is chemically equivalent to the amount of the substance in the sample, also known as the analyte. For acid-base titrations, this is the point at which the amount of acid exactly neutralizes the amount of base. The pH at this point is crucial as it provides insights into the properties of the acid or base being titrated.
In the context of base titrations, at the equivalence point, strong bases react completely with acids, often resulting in a noticeable change in the pH. The pH at the equivalence point is not always neutral (pH 7) but depends on the nature of the acid and base involved. With stronger bases, the equivalence point will have a higher pH because they form stronger conjugate acids, which are less acidic themselves.
Understanding the equivalence point provides clarity when analyzing titration curves, allowing for the identification of various characteristics of acids and bases. For example, if a titration results in a steep pH increase at the equivalence point, it confirms the strength of the interacting substances.

Strength of Bases

The strength of a base is determined by its ability to accept protons. A strong base will completely dissociate in an aqueous solution, meaning it releases a higher concentration of hydroxide ions ( OH^-). ) in the solution. Conversely, a weak base only partially dissociates, releasing fewer hydroxide ions.
During titration, the strength of a base can be observed by its pH level at the equivalence point. A higher pH indicates a stronger base because there are more hydroxide ions in the solution. This is due to the fact that stronger bases have stronger conjugate acids. These acids donate protons more readily, contributing to higher pH levels at equivalence.
In practical terms:

• A strong base completely dissociating might have a pH value over 12.
• A weak base might display a pH closer to neutral.

When examining different unknown bases, the one with the highest pH at the equivalence point is the strongest, while the one with the lowest pH is the weakest.

pH Scale

The pH scale is a measurement system used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14, where 7 is neutral, values less than 7 are acidic, and values greater than 7 are basic. The scale is logarithmic, which means each whole number step on the scale represents a tenfold increase or decrease in hydrogen ion concentration.
The pH level is significant in titrations because it helps identify how acidic or basic a solution is at any point in the process. For bases, higher pH values indicate stronger basic properties, as there are fewer hydrogen ions and more hydroxide ions present. The pH at the equivalence point during titration can therefore be a critical indicator of the strength of the unknown base.
When assessing unknown bases, it is crucial to understand that:

• A pH value closer to 14 suggests stronger basic properties.
• A lower pH value indicates weaker basic properties.

This understanding of the pH scale helps guide decisions in laboratory settings and in analytical chemistry when distinguishing between strong and weak bases.