Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

A base dissolved in water yields a solution with a hydroxyl ion concentration of \(0.05\) mol litre \(^{-1}\). The solution is (a) basic (b) acid (c) neutral (d) either (b) or (c)

Step-by-Step Solution

Answer

The solution is (a) basic.

Step 1: Understanding the Problem

We are tasked with determining the nature of a solution based on its hydroxyl ion concentration. Our options are that the solution is (a) basic, (b) acidic, (c) neutral, or (d) either (b) or (c).

Step 2: Recap of pH and Hydroxyl Ion Concentration

The pH and pOH of a solution are related to the concentrations of hydronium ions ( [H⁺] ) and hydroxyl ions ( [OH⁻] ) respectively. If a solution has a high concentration of OH⁻ ions, it is likely to be basic.

Step 3: Identifying the Solution Type

Hydroxyl ion concentration, [OH⁻] , is 0.05 mol / L. This is significantly greater than the [OH⁻] concentration in pure water, which has a concentration of 10^{-7} mol / L due to the autoionization of water. A higher concentration of OH⁻ ions implies that the solution is basic.

Step 4: Final Conclusion

Since the hydroxyl ion concentration is higher than in neutral water, the solution is indeed basic as this excess OH⁻ makes the solution basic.

Key Concepts

Hydroxyl Ion Concentration

In acid-base chemistry, the concentration of hydroxyl ions (\(OH^-\)) plays a critical role in determining the nature of a solution. Hydroxyl ions are negatively charged and occur when bases dissolve in water. The concentration of these ions is expressed in terms of moles per liter (mol/L), often denoted as \([OH^-]\).

For neutral water, the concentration of hydroxyl ions (\([OH^-]\)) is approximately \(10^{-7}\) mol/L because of water's autoionization. When the \([OH^-]\) exceeds this level, such as a concentration of \(0.05\) mol/L as in the problem, it indicates an influx of base into the water, resulting in a basic solution. This is a substantial increase compared to the neutral state, showcasing the dominance of base characteristics in the solution.

pH and pOH Relationship

The understanding of the pH and pOH relationship is pivotal in acid-base chemistry. The pH scale is used to express the acidity or basicity of a solution and ranges from 0 to 14. Similarly, pOH measures the concentration of hydroxyl ions. They are tied together through the equation:

• \[ pH + pOH = 14 \]

This relationship reveals the balance between hydrogen ions (\([H^+]\)) and hydroxyl ions (\([OH^-]\)) in a solution.

A lower pH value indicates a higher acidity (more hydrogen ions), while a lower pOH value depicts higher basicity (more hydroxyl ions). Since the solution in the exercise has a high concentration of \([OH^-]\), its pOH would be low, pushing the pH to be high; this confirms that the solution is strongly basic.

Solution Type Identification

Identifying a solution type involves determining its acidic, basic, or neutral nature based on ion concentration, specifically \([OH^-]\) and \([H^+]\). Typically:

• Acidic solutions have higher \([H^+]\) and lower \([OH^-]\).
• Basic solutions have higher \([OH^-]\) compared to \([H^+]\).
• Neutral solutions maintain equal concentrations, just like pure water.

Given the context of the provided hydroxyl ion concentration of \(0.05\) mol/L, which is noticeably higher than the neutral water's \(10^{-7}\) mol/L \([OH^-]\) concentration, the solution is characterized as basic. This is because the elevated presence of \([OH^-]\) outweighs any \([H^+]\) that might contribute to acidity, thereby firmly classifying it as a basic solution.