Problem 181 - A Complete Resource Book in Chemistry for JEE Main

Question

Separate solutions of four sodium salts \(\mathrm{NaW}, \mathrm{NaX}\), \(\mathrm{NaY}\) and \(\mathrm{NaZ}\) had \(\mathrm{pH} 7.0,9.0,10.0\) and \(11.0 \mathrm{respec}-\) tively. When each solution is \(0.1 \mathrm{M}\), the strongest acid is (a) \(\mathrm{HW}\) (b) HX (c) HY (d) \(\mathrm{HZ}\)

Step-by-Step Solution

Answer

The strongest acid is HW (option a).

Step 1: Understanding pH Levels

The pH of a solution is a measure of its acidity or basicity. A lower pH value indicates a stronger acid. Thus, we need to interpret the pH values of the sodium salt solutions given: - NaW: pH = 7.0 - NaX: pH = 9.0 - NaY: pH = 10.0 - NaZ: pH = 11.0. The solutions with lower pH are more acidic.

Step 2: Connecting Sodium Salts to Conjugate Acids

Each sodium salt, like NaW, is derived from its respective acid, for example HW. When the salt dissolves in water, the corresponding conjugate base is present. The pH of the solution indicates how weak the conjugate base is; a lower pH suggests a stronger conjugate acid. As per the pH values, NaW has the lowest pH.

Step 3: Identifying the Strongest Acid

Since NaW is the sodium salt with the lowest pH (pH = 7.0), it has the strongest conjugate acid component, HW. Hence, HW is the strongest acid.

Key Concepts

pH Levels

Understanding pH levels is crucial in acid-base chemistry. It helps you determine how acidic or basic a solution is.

The pH scale ranges from 0 to 14:

• A pH less than 7 indicates an acidic solution.
• A pH of 7 is neutral, like pure water.
• A pH greater than 7 indicates a basic (alkaline) solution.

In the context of sodium salt solutions, the solution with the lowest pH will be more acidic. Compare solutions with varying pH values to determine the relative acid strength of their conjugate acids.

In our scenario, NaW solution, with a pH of 7.0, is the most acidic among the sodium salts given.

Conjugate Acids

Conjugate acids are important to understand when dealing with salts in solutions. A conjugate acid forms when a base gains a hydrogen ion (proton).

When a salt such as NaX dissolves in water, it dissociates into its ions (Na⁺ and X^-). The X^- ion is the conjugate base of the original acid HX.

The strength of the conjugate acid is inversely related to the strength of its base:

• A weaker conjugate base results in a stronger conjugate acid.
• The strength of the acid can be inferred from the pH of the solution.
• Lower pH indicates a stronger conjugate acid.

Thus, NaW with the lowest pH suggests HW is the strongest conjugate acid in this context.

Sodium Salts

Sodium salts are compounds formed by the neutralization of an acid by sodium hydroxide. These salts help us understand the properties of their corresponding acids.

Let's take NaW as an example:

• It dissociates in water to form Na⁺ and W^- ions.
• W^- is the conjugate base of the acid HW.
• This equilibrium affects the overall pH of the solution.

Comparing sodium salts is a powerful way to deduce the relative strength of their corresponding acids based on the pH values of their solutions. Sodium salts with lower solution pH have stronger corresponding acids.

Acid Strength

The strength of an acid is determined by its ability to donate protons to a base. Stronger acids completely dissociate in water, releasing more hydrogen ions.

Here are key points about acid strength:

• Acid strength is inversely related to the pH value of its salt solution; lower pH suggests a stronger acid.
• The stronger an acid, the weaker its conjugate base.
• HW, the acid associated with NaW, will have a higher propensity to donate protons compared to HX, HY, and HZ, as it leads to the lowest pH solution.

In summary, recognizing patterns in pH variations across sodium salt solutions allows us to rank the acids from strongest to weakest.