Problem 42 - Chemistry: An Atoms-Focused Approach

Question

The \(\mathrm{pH}\) of a solution of a strong monoprotic acid is lower than the pH of an equal concentration of a weak monoprotic acid, yet equal volumes of both require the same volume of basic titrant to reach the equivalence point. Explain why.

Step-by-Step Solution

Answer

Answer: During titration, both strong and weak monoprotic acids release one mole of H+ ions per mole of acid. This means that the amount of H+ ions that need to be neutralized by the basic titrant is the same for both strong and weak monoprotic acids with equal concentrations, regardless of their pH values. As a result, they require the same volume of basic titrant to reach the equivalence point.

Step 1: Understand strong and weak acids

A strong monoprotic acid is an acid that fully ionizes in a solution, meaning that it releases all of its hydrogen ions (H+) when dissolved. A weak monoprotic acid, on the other hand, partially ionizes in a solution and only releases some of its hydrogen ions.

Step 2: Defining pH

The pH of a solution is a measure of the concentration of hydrogen ions (H+) in the solution. The pH scale ranges from 0 to 14, with solutions of pH less than 7 being acidic, pH equal to 7 being neutral, and pH greater than 7 being basic.

Step 3: Comparing pH of strong and weak acids

Since strong monoprotic acids fully ionize and release more hydrogen ions (H+) compared to weak monoprotic acids, the concentration of H+ ions in a solution of a strong acid is higher than that in a solution of a weak acid with equal concentrations. This results in a lower pH value for the strong monoprotic acid, indicating a more acidic solution.

Step 4: Titration and reaching the equivalence point

Titration is a technique used to determine the concentration of a solution by adding a known amount of a solution with known concentration (the titrant) to a solution with an unknown concentration. The equivalence point is reached when the amount of titrant added is stoichiometrically equal to the amount of the unknown solution.

Step 5: Explanation for equal volume of basic titrant

In the case of titrating monoprotic acids, whether strong or weak, they release one mole of H+ ions per mole of acid. Therefore, during the titration, the amount of H+ ions that need to be neutralized by the basic titrant is the same for both strong and weak monoprotic acids with equal concentrations. This is the reason why equal volumes of both types of acids require the same volume of basic titrant to reach the equivalence point, despite having different pH values.