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) HW (b) \(\mathrm{HX}\) (c) HY (d) \(\mathrm{HZ}\)

Step-by-Step Solution

Answer

The strongest acid is (a) HW.

Step 1: Understand the Relationship Between Acid Strength and pH

The pH of a solution indicates its acidity or basicity. A lower pH means a stronger acid is present, while a higher pH indicates a weaker acid or stronger base. Therefore, we will compare the pH values of the solutions to determine the strength of their corresponding acids.

Step 2: Analyze the Given pH Values

We have four solutions with sodium salts having pH values: NaW (pH 7.0), NaX (pH 9.0), NaY (pH 10.0), and NaZ (pH 11.0). Lower pH implies a stronger acid. Thus, the salt with the smallest pH relates to the strongest acid among the given options.

Step 3: Identify the Strongest Acid Based on pH

Given the pH values, the solution with pH 7.0, which corresponds to NaW, is in a neutral pH range, suggesting a weakly acidic or neutral nature. Since it has the lowest pH among the given samples, HW is the strongest acid when dissociated in solution.

Key Concepts

pH scale

The pH scale is a simple yet powerful tool to measure the acidity or alkalinity of a solution. It ranges from 0 to 14, where a pH of 7 is considered neutral. Values below 7 indicate an acidic environment, while numbers above 7 represent a basic (or alkaline) environment. Each unit change on the pH scale represents a tenfold change in acidity or basicity. For instance, a pH of 6 is ten times more acidic than a pH of 7.

Understanding the pH scale is crucial when analyzing substances as it directly relates to their acid-base properties. Strong acids have low pH values, usually between 0 and 3, and strong bases have high pH values, generally from 11 to 14. In the case of sodium salts, their pH values can tell us a lot about their resulting acids when dissociated in water.

sodium salts

Sodium salts, such as NaW, NaX, NaY, and NaZ, are compounds created when sodium ions (Na⁺) neutralize the effects of acids. The identity of these acids becomes apparent when the sodium salt dissolves in water. Upon dissociation, the anions (negatively charged ions) may exhibit basic properties, influencing the solution's pH.

For example, a sodium salt like NaW producing a solution with a pH of 7 suggests the original acid (HW) might be weak since it does not lower the pH much below neutral. Conversely, if a sodium salt results in a higher pH, the original acid form is much weaker, allowing the anion to act more like a base. This relationship is key to understanding how sodium salts behave in aqueous solutions.

acid-base properties

Acid-base properties are fundamental to understanding chemical reactions in solutions. An acid is a substance that can donate hydrogen ions (H⁺) to a solution, lowering the overall pH and making it more acidic. A base, on the other hand, can accept hydrogen ions, often raising the pH and making the solution more alkaline.

When analyzing sodium salts, the nature of the original acid informs us about its acid-base properties. A salt leading to a neutral or lower pH indicates a stronger acid, as it allows the solution to hover closer to the acidic side of the scale. Conversely, higher pH levels associated with sodium salts indicate weaker acids, with their corresponding anions behaving more like bases.

solution concentration (Molarity)

Molarity is a term used to describe the concentration of a solution. It is expressed in moles of solute per liter of solution, denoted as M. In a problem statement referencing a 0.1 M solution, it implies that 0.1 moles of the sodium salt are dissolved in every liter of solution.

This concentration plays a significant role when evaluating acid strength. Even slight variations can influence pH, but when comparing substances of the same molarity, acid strength becomes apparent due to differences in how each acid dissociates in the water. Therefore, sodium salts, when in 0.1 M solutions, can be evaluated reliably for pH steps, given that all are at equivalent concentrations.