Question

Separate solutions of NaW, NaX, NaY and NaZ, each of concentrations \(0.1 \mathrm{M}\), has \(\mathrm{pH} 7.0,9.0,10.0\) and \(11.0\) respectively, at \(25^{\circ} \mathrm{C}\). The strongest acid among these is (a) \(\mathrm{NaW}\) (b) NaX (c) NaY (d) \(\mathrm{NaZ}\)

Short answer

The strongest acid among NaW, NaX, NaY, and NaZ is (a) NaW, since it has the lowest pH, indicating that it comes from the strongest acid.

Step-by-step solution

Understanding pH

pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a pH less than 7, neutral solutions have a pH of 7, and basic solutions have a pH greater than 7. The given solutions all have pH values above 7, indicating that they are basic solutions, as these are solutions of salts formed from the neutralization of acids and bases. The strength of the acid that formed the salt can be determined indirectly by the pH of the resulting solution.

Determining the Acidity

The pH of a solution is inversely related to the concentration of hydronium ions (H3O+) which is directly related to the strength of the acid that formed the salt. The lower the pH, the higher the concentration of H3O+ ions, and the stronger the acid. So, we compare the pH values of each solution to determine which one comes from the strongest acid.

Comparing the Given pH Values

We are given that NaW has a pH of 7.0, NaX has a pH of 9.0, NaY has a pH of 10.0, and NaZ has a pH of 11.0. Since the strongest acid would result in the formation of a salt with the lowest pH in a basic solution, we identify NaW with a pH of 7.0 as the salt formed from the strongest acid.

Key concepts

The pH Scale

The pH scale is a measure of how acidic or basic a solution is, with values ranging from 0 to 14. At room temperature (25°C), a pH value of 7 is considered neutral, which is the pH of pure water. Solutions with a pH less than 7 are acidic, while those with a pH greater than 7 are basic or alkaline. Each unit change in pH represents a tenfold change in acidity or basicity, making the pH scale logarithmic.

For example, a solution with a pH of 3 is ten times more acidic than one with a pH of 4 and a hundred times more acidic than a solution with a pH of 5. This scale helps in comparing the relative strengths of acidic and basic solutions and is crucial for many chemical processes and biological systems.

Hydronium Ion Concentration

Hydronium ions (( H3O+)) are the positively charged ions formed when an acid dissolves in water. The concentration of these ions in a solution determines its acidity. In technical terms, pH is the negative logarithm of the molar concentration of hydronium ions:

( pH = -log[H3O+] )

As the concentration of hydronium ions increases, the pH value decreases, making the solution more acidic. Conversely, as the concentration decreases, the pH value increases, indicating a more basic solution. Thus, by measuring the pH, we indirectly measure the concentration of hydronium ions in a solution.

Acid-Base Neutralization

Acid-base neutralization is a chemical reaction in which an acid and a base react to form water and a salt. This reaction is important because it is often used to determine the strength of an acid or base, and it can also neutralize potentially harmful substances. The resulting pH of the solution after neutralization can indicate the relative strengths of the starting acid and base.

For example, if a strong acid is neutralized by a weak base, the resulting solution is slightly acidic. Conversely, if a strong base neutralizes a weak acid, the resulting solution is slightly basic. The pH value of the salt solutions created from these neutralization reactions can be measured to assess the strength of the original acid or base.

Strength of Acids

The strength of an acid can be understood in terms of its ability to donate protons (H+), and is often reflected in its dissociation constant (Ka). Strong acids almost completely dissociate in water, releasing all their protons, thereby increasing the concentration of hydronium ions ((H3O+)), and resulting in a lower pH of the solution. Weak acids only partially dissociate, contributing fewer hydronium ions to the solution.

Determining the strength of an acid helps predict the direction and extent of acid-base reactions. It is crucial for tasks ranging from buffer preparation in laboratories to the efficient processing of food in industries. In the context of the provided exercise, the salt NaW, which has a pH closest to neutral (7), is produced by the neutralization of the strongest acid among the given options, as it releases the most protons.